Cell Motility and the Cytoskeleton 15:67-70 (1990)
Views and Reviews
Role of Microtubules in the Organisation of the Golgi Apparatus Thomas
E.Kreis
European Molecular Biology Laboratory, Heidelberg, Federal Republic of Germany
It has become increasingly evident that the microtubule network plays a direct and crucial role in the organisation of an optimal distribution of cytoplasmic organelles and in the vesicle-mediated traffic in between these membrane-bounded compartments. The Golgi apparatus is a key organelle involved in intracellular membrane traffic, and an interaction of this organelle with microtubules has been shown by several studies [cf. Thyberg and Moskalewski, 19851. The Golgi apparatus consists of an interconnected network of stacks of flattened cisternae and tubular structures. It is involved in the processing, maturation, and sorting of secretory and membrane proteins, as well as in glycolipid metabolism. Specific enzymes located in the different cisternae sequentially modify these molecules as they move along their biosynthetic pathway [cf. Kornfeld and Kornfeld, 19851. The restricted distribution of particular enzymes to specific Golgi cisternae defines a functional polarity of the Golgi apparatus [cf. Dunphy and Rothman, 19851. The Golgi apparatus also exhibits distinct morphological polarity. The cis-cisternae comprises an extensive, fenestrated, ribbon-like structure and is often apposed to the endoplasmic reticulum and the nuclear envelope [Rambourg et al., 1974; cf. Farquhar and Palade, 19811. Vesicular carriers from the endoplasmic reticulum enter the Golgi apparatus here [Farquhar and Palade, 19811. Different classes of transport vesicles, like secretory vesicles, and vesicles carrying material to the plasma membrane or to the endocytic organelles, bud from the largely uninterrupted truns-cisternae on the opposite side of the Golgi complex [Palade, 19751. The Golgi apparatus co-localizes with the minus ends of interphase microtubules, which are usually associated with the microtubule-organizing center. Evidence for an interaction of elements of the Golgi apparatus with microtubules during establishment and maintenance of 0 1990 Wiley-Liss, Inc.
this particular position is manyfold. Both the Golgi apparatus and the microtubule-organizing center change their location in a coordinate way during cell locomotion [cf. Singer and Kupfer, 19861 and cell differentiation [Tassin et al., 1985; Bacallao et al., 19891. The Golgi apparatus fragments into tubulo-vesicular clusters at the onset of mitosis, when the interphase microtubules depolymerize and the mitotic spindle forms [Lucocq et al., 19891. Furthermore, agents that alter the distribution of interphase microtubules have profound effects on integrity and location of the Golgi apparatus [Thyberg and Moskalewski, 19851. For example, treatment of cells with nocodazole or similar drugs that induce complete depolymerization of interphase microtubules leads to fragmentation of the Golgi apparatus into distinct elements, which randomly disperse throughout the cytoplasm. These dispersed Golgi elements reaggregate upon removal of the drug and subsequent microtubule repolymerisation. Analysis of vitally stained cells reveals that Golgi elements move along microtubules toward their minus ends during reclustering, and neither intermediate filaments nor microfilaments are involved in this process [Ho et al., 19891. Taken together, these studies indicate that intact interphase microtubules play an essential role in maintaining the structural integrity and the location of the Golgi apparatus. A likely candidate for the motor protein involved in the translocation of the Golgi elements along microtubules is cytoplasmic dynein. Cytoplasmic dynein is a minus end-directed microtubule-dependent motor protein
Accepted September 28, 1989 Address reprint requests t o Thomas E. Kreis, European Molecular Biology Laboratory, Meyerhofstrasse I 6900 Heidelberg. FRG.
.
68
Kreis
I
endoplasmic reticulum
plasma membrane
I
I
Fig. 1
[Paschal et al., 19871, and it has been shown to be capable of moving organelles along microtubules in extracts of tissue culture cells [Schroer et al., 19891. Therefore, it may play a role in reclustering of Golgi elements following mitosis or removal of nocodazole. Movement of various vesicular organelles or membranes of the rough endoplasmic reticulum along microtubules has been reconstituted in vitro [Dabora and Sheetz, 1988; Vale and Hotani, 1988; cf. also Vale, 19871. Reconstitution of movement of Golgi elements along microtubules in vitro should help to elucidate the molecular components involved in these processes. Clearly, membranes of the Golgi apparatus must associate with the microtubules. Interesting questions concern the nature and location of this interaction of microtubules with the Golgi elements. In contrast to vesicles, which are simple, spherical structures, the moving Golgi elements consist of a variety of membrane domains [Ho et al., 19891. Microtubules may interact just with the rims of the stacked cisternae or along the lateral surface of the cisternae, and regions more cis- or translocated on the organelle may exhibit preferential binding properties for microtubules. Furthermore, interactions other than those mediating movement induced by microtubule-based motors may be involved in anchoring the Golgi elements in the region of the microtubule minus ends and the microtubule-organizing center. The proteins mediating these interactions will now have to be identified and characterized. Two proteins of Mr 110,000 [Allan and Kreis, 19861 and Mr 58,000 [Bloom and Brashear, 19891, which may represent good
candidates for a new class of proteins linking elements of the Golgi apparatus to microtubules have been identified recently. Immunofluorescence staining with monoclonal antibodies recognizing either of these two proteins labels the Golgi apparatus in tissue culture cells. Both proteins are associated with the cytoplasmic face of Golgi membranes [Allan and Kreis, 1986; Bloom and Brashear, 19891. Furthermore, the two proteins bind to microtubules in vitro. Additionally, the Mr 58,000 protein stimulates polymerization of microtubules in vitro [Bloom and Brashear, 19891. Thus, both proteins may be directly involved in the interaction of the Golgi apparatus with microtubules. Appropriate functional assays will be necessary to further characterize the precise roles of these two proteins. The Golgi apparatus plays a crucial role in intracellular membrane traffic. In a simplified model (see Fig. I ) , it may be compared to a central station located at one end of the intracellular tracks (minus ends of microtubules), which support movement of membrane-bounded containers (vesicles or organelles). These tracks distributc cargo to the cell perimeter (e.g., secretory or plasma membrane components), and they guide “incoming” movement of material (e.g., in endosomes) toward the region of the microtubule-organizing center and the Golgi apparatus. Because in many cells both Golgi apparatus, as well as late endosomes and lysosomes colocalize in the area of the microtubule-organizing center [cf. Kreis et al., 19881, it is tempting to speculate that the spatial apposition of these two compartments facilitates intercompartmental transport. Lysosomal enzymes, for example, are sorted in the Golgi apparatus by specific receptors into coated vesicles, which deliver them to the endosomal/lysosomal organelles [cf. Kornfeld, 19871. Furthermore, a fraction of endocytosed membrane protein is recycled via the trans-region of the Golgi apparatus back to the plasma membrane. Thus, the area of the microtubule-organizing center may provide a meeting place (shaded area in Fig. 1) for linking together the endocytic and exocytic membrane pathways. Dispersed elements of the Golgi apparatus move retrogradely (i.e., minus end-directed) along microtubules toward the area of the microtubule-organizing center [Ho et al., 19891. Furthermore, microtubules may continuously relocate Golgi elements toward this region of the cell, ensuring maintenance of a compact Golgi complex. Additional microtubule-independent factors (“glue-factors”) may be involved in anchoring the Golgi elements (as well as late endosomes and lysosomes) to this area (shaded area in Fig. 1). Secretory vesicles, on the other hand, bud from the trans-Golgi network and move rapidly in an anterograde direction toward the cell periphery [Kreis et al., 19891 even though the transGolgi network, from which these vesicles are derived,
Microtubules in Organisation of Golgi Apparatus
moves together with the Golgi complex in the opposite direction. Two intriguing models can be considered to explain this apparently paradoxical situation. Microtubule plus end-directed motors [like kinesin; Vale et al., 19851 associate exclusively with the rrans-Golgi network-derived exocytic carrier vesicles via receptors, which may cluster into the regions of the budding vesicles and/or may be activated upon budding. Dynein-like motors bind, in this model, constitutively to the membranes of the Golgi apparatus proper and are responsible for continuous relocation of the Golgi elements toward the area of the microtubule-organizing center. Alternatively, gradients in the densities of receptors might exist across the Golgi apparatus, with the microtubule plus and microtubule minus end-directed motors accumulating at the trans- and cis-side, respectively. The minus end-directed motors associated with the Golgi membranes are dominant, and the cisternae of the Golgi apparatus are kept in stacks by intercisternal adhesive factors (see Fig. 1). Once a secretory vesicle buds off from the trans-Golgi network, it will eventually associate with a microtubule nearby, and the kinesin(-like) motors will move it to the cell periphery. Elements of the Golgi apparatus and endocytic organelles move toward the minus ends of microtubules. Are identical motor proteins (i.e., cytoplasmic dynein) responsible for these movements, or do different, albeit analogous, motors associated with Golgi elements and endosomes, respectively? The latter possibility is favored by the finding that endosomes, but not Golgi elements, can reverse their direction of movement along microtubules upon acidification of the cytosol [Heuser, 19891. In conclusion, the Golgi apparatus is a dynamic organelle built up of distinct membranous subunits. Microtubules play an important role in the dynamic organization of the structure and location of these Golgi elements. As yet, very little is known about the molecular components involved in maintaining the correct location of the Golgi apparatus in the area of the microtubuleorganizing center and in mediating binding of membranes of the Golgi apparatus to microtubules. Further studies are necessary to identify and biochemically characterize the proteins that are involved in these processes. Functional in vitro assays for binding of Golgi or Golgiderived elements to microtubules, or for movement of these membrane structures along microtubules can then be applied to elucidate the role of these proteins in the dynamic interactions of Golgi membranes with microtubules.
69
drawing the Figure, and my collegues in the laboratory for many stimulating discussions. REFERENCES Allan, V.J., and Kreis, T.E. (1986): A microtubule-binding protein associated with membranes of the Golgi apparatus. J. Cell Biol. 103:2229-2239. Bacallao, R., Antony, C., Dotti, C., Karsenti, E.. Stelzer, E.H.K.. and Simons, K. (1989): The subcellular organisation of MDCK cells during the formation of a polarized epithelium. J . Cell Biol., in press. Bloom, G.S., and Brashear, T.A. (1989): A novel 58-kDa protein associates with the Golgi apparatus and microtubules. J . B i d . Chem. 264:16083-16092. Dabora, S.L., and Sheetz, M.P. (1988): The microtubule-dependent formation of a tubulovesicular network with characteristics of the ER from cultured cell extracts. Cell 54:27-35. Dunphy, W.G., and Rothman, J.E. (1985): Compartmental organization of the Golgi stack. Cell 42: 13-2 I. Farquhar. M.G., and Palade, G.F. (1981): The Golgi apparatus (comartifact to center stage. J . Cell plex)-( 1954-1981)-from Biol. 9 I:77s-I03s. Heuser, J. (1989): Changes in lysosome shape and distribution correlated with changes in cytoplasmic pH. J. Cell Biol. 108:855Kb4. Ho, W.C., Allan, V.J., van Meer, G., Berger, E.G., and Kreis, T.E. (1989): Reclustering of scattered Golgi elements occurs along microtubules. Eur. J . Cell Biol. 48:250-263. Kornfeld, S. (1987): Trafficking of lysosomal enzymes. FASEB J. I:462-468. Kornfeld, R., and Kornfeld, S. (1985): Assembly of asparaginelinked oligosaccharides. Annu. Rev. Biochem. 54:63 1-664. Kreis, T.E., Allan, V.J., Matteoni, R., and Ho, W.C. (1988): Interaction of elements of the Golgi apparatus with microtubules. Protoplasma 145: 153-159. Kreis, T.E., Matteoni, R., Hollinshead, M., and Tooze, J. (1989): Secretory granules and endosomes show saltatory movement biased to the anterograde and retrograde directions, respectively, along microtubules in AtT20 cells. Eur. J. Cell Biol. 49: 1 28- 1 39. Lucocq, J.M., Berger, E.M., and Warren, G. (1989): Mitotic Golgi fragments in HeLa cells and their role in the reassembly pathway. J . Cell Biol. 109:463-474. Palade, G . (1975): Intracellular aspects of the process of protein synthesis. Science 187:347-358. Paschal, B.M., Shpetner, H.S., and Vallee, R.B. (1987): M A P l C is a microtubule activated ATPase which translocates microtubules in vitro and has dynein-like properties. J . Cell Biol. 105: I 273-1 282. Rambourg, A , , Clerniont, Y . , and Marrano, A. (1974): Three-dimensional structure o f osmium impregnated Golgi apparatus as seen by high voltage electron microscope. Am. J . Anat. 140: 27-46. Schroer, T.A., Steuer, E.R., and Sheetz, M.P. (1989): Cytoplasniic dynein is a minus end-directed motor for membranous organelles. Cell 56937-946. Singer. S.J., and Kupfer. A. (1986): The directed migration 0 1 eu-
70
Kreis
Thyberg, J., and Moskalewski, S. (1985): Microtubules and the organization of the Golgi complex. Exp. Cell Res. 159:1-16. Vale, R.D. (1987): lntracellular transport using microtubule-based motors. Annu. Rev. Cell Biol. 3:347-378. Vale, R.D., and Hotani, H. (1988): Formation of membrane networks
i n vitro by kinesin-driven microtubule movements. J . Cell Biol. 107:2233-2241. Vale, R.D., Reese, T.S., and Sheetz, M.P. (1985): Identification of a novel force-generating protein, kinesin, involved in microtubule-based motility. Cell 42:39-50.
Views and Reviews
Role of Microtubules in the Organisation of the Golgi Apparatus Thomas
E.Kreis
European Molecular Biology Laboratory, Heidelberg, Federal Republic of Germany
It has become increasingly evident that the microtubule network plays a direct and crucial role in the organisation of an optimal distribution of cytoplasmic organelles and in the vesicle-mediated traffic in between these membrane-bounded compartments. The Golgi apparatus is a key organelle involved in intracellular membrane traffic, and an interaction of this organelle with microtubules has been shown by several studies [cf. Thyberg and Moskalewski, 19851. The Golgi apparatus consists of an interconnected network of stacks of flattened cisternae and tubular structures. It is involved in the processing, maturation, and sorting of secretory and membrane proteins, as well as in glycolipid metabolism. Specific enzymes located in the different cisternae sequentially modify these molecules as they move along their biosynthetic pathway [cf. Kornfeld and Kornfeld, 19851. The restricted distribution of particular enzymes to specific Golgi cisternae defines a functional polarity of the Golgi apparatus [cf. Dunphy and Rothman, 19851. The Golgi apparatus also exhibits distinct morphological polarity. The cis-cisternae comprises an extensive, fenestrated, ribbon-like structure and is often apposed to the endoplasmic reticulum and the nuclear envelope [Rambourg et al., 1974; cf. Farquhar and Palade, 19811. Vesicular carriers from the endoplasmic reticulum enter the Golgi apparatus here [Farquhar and Palade, 19811. Different classes of transport vesicles, like secretory vesicles, and vesicles carrying material to the plasma membrane or to the endocytic organelles, bud from the largely uninterrupted truns-cisternae on the opposite side of the Golgi complex [Palade, 19751. The Golgi apparatus co-localizes with the minus ends of interphase microtubules, which are usually associated with the microtubule-organizing center. Evidence for an interaction of elements of the Golgi apparatus with microtubules during establishment and maintenance of 0 1990 Wiley-Liss, Inc.
this particular position is manyfold. Both the Golgi apparatus and the microtubule-organizing center change their location in a coordinate way during cell locomotion [cf. Singer and Kupfer, 19861 and cell differentiation [Tassin et al., 1985; Bacallao et al., 19891. The Golgi apparatus fragments into tubulo-vesicular clusters at the onset of mitosis, when the interphase microtubules depolymerize and the mitotic spindle forms [Lucocq et al., 19891. Furthermore, agents that alter the distribution of interphase microtubules have profound effects on integrity and location of the Golgi apparatus [Thyberg and Moskalewski, 19851. For example, treatment of cells with nocodazole or similar drugs that induce complete depolymerization of interphase microtubules leads to fragmentation of the Golgi apparatus into distinct elements, which randomly disperse throughout the cytoplasm. These dispersed Golgi elements reaggregate upon removal of the drug and subsequent microtubule repolymerisation. Analysis of vitally stained cells reveals that Golgi elements move along microtubules toward their minus ends during reclustering, and neither intermediate filaments nor microfilaments are involved in this process [Ho et al., 19891. Taken together, these studies indicate that intact interphase microtubules play an essential role in maintaining the structural integrity and the location of the Golgi apparatus. A likely candidate for the motor protein involved in the translocation of the Golgi elements along microtubules is cytoplasmic dynein. Cytoplasmic dynein is a minus end-directed microtubule-dependent motor protein
Accepted September 28, 1989 Address reprint requests t o Thomas E. Kreis, European Molecular Biology Laboratory, Meyerhofstrasse I 6900 Heidelberg. FRG.
.
68
Kreis
I
endoplasmic reticulum
plasma membrane
I
I
Fig. 1
[Paschal et al., 19871, and it has been shown to be capable of moving organelles along microtubules in extracts of tissue culture cells [Schroer et al., 19891. Therefore, it may play a role in reclustering of Golgi elements following mitosis or removal of nocodazole. Movement of various vesicular organelles or membranes of the rough endoplasmic reticulum along microtubules has been reconstituted in vitro [Dabora and Sheetz, 1988; Vale and Hotani, 1988; cf. also Vale, 19871. Reconstitution of movement of Golgi elements along microtubules in vitro should help to elucidate the molecular components involved in these processes. Clearly, membranes of the Golgi apparatus must associate with the microtubules. Interesting questions concern the nature and location of this interaction of microtubules with the Golgi elements. In contrast to vesicles, which are simple, spherical structures, the moving Golgi elements consist of a variety of membrane domains [Ho et al., 19891. Microtubules may interact just with the rims of the stacked cisternae or along the lateral surface of the cisternae, and regions more cis- or translocated on the organelle may exhibit preferential binding properties for microtubules. Furthermore, interactions other than those mediating movement induced by microtubule-based motors may be involved in anchoring the Golgi elements in the region of the microtubule minus ends and the microtubule-organizing center. The proteins mediating these interactions will now have to be identified and characterized. Two proteins of Mr 110,000 [Allan and Kreis, 19861 and Mr 58,000 [Bloom and Brashear, 19891, which may represent good
candidates for a new class of proteins linking elements of the Golgi apparatus to microtubules have been identified recently. Immunofluorescence staining with monoclonal antibodies recognizing either of these two proteins labels the Golgi apparatus in tissue culture cells. Both proteins are associated with the cytoplasmic face of Golgi membranes [Allan and Kreis, 1986; Bloom and Brashear, 19891. Furthermore, the two proteins bind to microtubules in vitro. Additionally, the Mr 58,000 protein stimulates polymerization of microtubules in vitro [Bloom and Brashear, 19891. Thus, both proteins may be directly involved in the interaction of the Golgi apparatus with microtubules. Appropriate functional assays will be necessary to further characterize the precise roles of these two proteins. The Golgi apparatus plays a crucial role in intracellular membrane traffic. In a simplified model (see Fig. I ) , it may be compared to a central station located at one end of the intracellular tracks (minus ends of microtubules), which support movement of membrane-bounded containers (vesicles or organelles). These tracks distributc cargo to the cell perimeter (e.g., secretory or plasma membrane components), and they guide “incoming” movement of material (e.g., in endosomes) toward the region of the microtubule-organizing center and the Golgi apparatus. Because in many cells both Golgi apparatus, as well as late endosomes and lysosomes colocalize in the area of the microtubule-organizing center [cf. Kreis et al., 19881, it is tempting to speculate that the spatial apposition of these two compartments facilitates intercompartmental transport. Lysosomal enzymes, for example, are sorted in the Golgi apparatus by specific receptors into coated vesicles, which deliver them to the endosomal/lysosomal organelles [cf. Kornfeld, 19871. Furthermore, a fraction of endocytosed membrane protein is recycled via the trans-region of the Golgi apparatus back to the plasma membrane. Thus, the area of the microtubule-organizing center may provide a meeting place (shaded area in Fig. 1) for linking together the endocytic and exocytic membrane pathways. Dispersed elements of the Golgi apparatus move retrogradely (i.e., minus end-directed) along microtubules toward the area of the microtubule-organizing center [Ho et al., 19891. Furthermore, microtubules may continuously relocate Golgi elements toward this region of the cell, ensuring maintenance of a compact Golgi complex. Additional microtubule-independent factors (“glue-factors”) may be involved in anchoring the Golgi elements (as well as late endosomes and lysosomes) to this area (shaded area in Fig. 1). Secretory vesicles, on the other hand, bud from the trans-Golgi network and move rapidly in an anterograde direction toward the cell periphery [Kreis et al., 19891 even though the transGolgi network, from which these vesicles are derived,
Microtubules in Organisation of Golgi Apparatus
moves together with the Golgi complex in the opposite direction. Two intriguing models can be considered to explain this apparently paradoxical situation. Microtubule plus end-directed motors [like kinesin; Vale et al., 19851 associate exclusively with the rrans-Golgi network-derived exocytic carrier vesicles via receptors, which may cluster into the regions of the budding vesicles and/or may be activated upon budding. Dynein-like motors bind, in this model, constitutively to the membranes of the Golgi apparatus proper and are responsible for continuous relocation of the Golgi elements toward the area of the microtubule-organizing center. Alternatively, gradients in the densities of receptors might exist across the Golgi apparatus, with the microtubule plus and microtubule minus end-directed motors accumulating at the trans- and cis-side, respectively. The minus end-directed motors associated with the Golgi membranes are dominant, and the cisternae of the Golgi apparatus are kept in stacks by intercisternal adhesive factors (see Fig. 1). Once a secretory vesicle buds off from the trans-Golgi network, it will eventually associate with a microtubule nearby, and the kinesin(-like) motors will move it to the cell periphery. Elements of the Golgi apparatus and endocytic organelles move toward the minus ends of microtubules. Are identical motor proteins (i.e., cytoplasmic dynein) responsible for these movements, or do different, albeit analogous, motors associated with Golgi elements and endosomes, respectively? The latter possibility is favored by the finding that endosomes, but not Golgi elements, can reverse their direction of movement along microtubules upon acidification of the cytosol [Heuser, 19891. In conclusion, the Golgi apparatus is a dynamic organelle built up of distinct membranous subunits. Microtubules play an important role in the dynamic organization of the structure and location of these Golgi elements. As yet, very little is known about the molecular components involved in maintaining the correct location of the Golgi apparatus in the area of the microtubuleorganizing center and in mediating binding of membranes of the Golgi apparatus to microtubules. Further studies are necessary to identify and biochemically characterize the proteins that are involved in these processes. Functional in vitro assays for binding of Golgi or Golgiderived elements to microtubules, or for movement of these membrane structures along microtubules can then be applied to elucidate the role of these proteins in the dynamic interactions of Golgi membranes with microtubules.
69
drawing the Figure, and my collegues in the laboratory for many stimulating discussions. REFERENCES Allan, V.J., and Kreis, T.E. (1986): A microtubule-binding protein associated with membranes of the Golgi apparatus. J. Cell Biol. 103:2229-2239. Bacallao, R., Antony, C., Dotti, C., Karsenti, E.. Stelzer, E.H.K.. and Simons, K. (1989): The subcellular organisation of MDCK cells during the formation of a polarized epithelium. J . Cell Biol., in press. Bloom, G.S., and Brashear, T.A. (1989): A novel 58-kDa protein associates with the Golgi apparatus and microtubules. J . B i d . Chem. 264:16083-16092. Dabora, S.L., and Sheetz, M.P. (1988): The microtubule-dependent formation of a tubulovesicular network with characteristics of the ER from cultured cell extracts. Cell 54:27-35. Dunphy, W.G., and Rothman, J.E. (1985): Compartmental organization of the Golgi stack. Cell 42: 13-2 I. Farquhar. M.G., and Palade, G.F. (1981): The Golgi apparatus (comartifact to center stage. J . Cell plex)-( 1954-1981)-from Biol. 9 I:77s-I03s. Heuser, J. (1989): Changes in lysosome shape and distribution correlated with changes in cytoplasmic pH. J. Cell Biol. 108:855Kb4. Ho, W.C., Allan, V.J., van Meer, G., Berger, E.G., and Kreis, T.E. (1989): Reclustering of scattered Golgi elements occurs along microtubules. Eur. J . Cell Biol. 48:250-263. Kornfeld, S. (1987): Trafficking of lysosomal enzymes. FASEB J. I:462-468. Kornfeld, R., and Kornfeld, S. (1985): Assembly of asparaginelinked oligosaccharides. Annu. Rev. Biochem. 54:63 1-664. Kreis, T.E., Allan, V.J., Matteoni, R., and Ho, W.C. (1988): Interaction of elements of the Golgi apparatus with microtubules. Protoplasma 145: 153-159. Kreis, T.E., Matteoni, R., Hollinshead, M., and Tooze, J. (1989): Secretory granules and endosomes show saltatory movement biased to the anterograde and retrograde directions, respectively, along microtubules in AtT20 cells. Eur. J. Cell Biol. 49: 1 28- 1 39. Lucocq, J.M., Berger, E.M., and Warren, G. (1989): Mitotic Golgi fragments in HeLa cells and their role in the reassembly pathway. J . Cell Biol. 109:463-474. Palade, G . (1975): Intracellular aspects of the process of protein synthesis. Science 187:347-358. Paschal, B.M., Shpetner, H.S., and Vallee, R.B. (1987): M A P l C is a microtubule activated ATPase which translocates microtubules in vitro and has dynein-like properties. J . Cell Biol. 105: I 273-1 282. Rambourg, A , , Clerniont, Y . , and Marrano, A. (1974): Three-dimensional structure o f osmium impregnated Golgi apparatus as seen by high voltage electron microscope. Am. J . Anat. 140: 27-46. Schroer, T.A., Steuer, E.R., and Sheetz, M.P. (1989): Cytoplasniic dynein is a minus end-directed motor for membranous organelles. Cell 56937-946. Singer. S.J., and Kupfer. A. (1986): The directed migration 0 1 eu-
70
Kreis
Thyberg, J., and Moskalewski, S. (1985): Microtubules and the organization of the Golgi complex. Exp. Cell Res. 159:1-16. Vale, R.D. (1987): lntracellular transport using microtubule-based motors. Annu. Rev. Cell Biol. 3:347-378. Vale, R.D., and Hotani, H. (1988): Formation of membrane networks
i n vitro by kinesin-driven microtubule movements. J . Cell Biol. 107:2233-2241. Vale, R.D., Reese, T.S., and Sheetz, M.P. (1985): Identification of a novel force-generating protein, kinesin, involved in microtubule-based motility. Cell 42:39-50.
