Summary The assembly of the major small nuclear ribonucleoprotein (snRNP)? particles begins in the cytoplasm where large pools of common core proteins are preassembled in several RNA-free intermediate particles. Newly synthesized snRNAs transiently enter the cytoplasm and complex with core particles to form pre-snRNP particles. Subsequently, the cap structure at the 5' end of the snRNA is hypermethylated. The resulting trimethylguanosine (TMG) cap is an integral part of the nuclear localization signal for snRNP particles and the pre-snRNP particles are rapidly transported into the nucleus. SnRNP particles mature when snRNA-specific proteins complex with the particles, in some cases, just before or during nuclear transport, but in most instances after the particles are in the nucleus. In addition, U6 snRNA hybridizes with U4 snRNA to form a U4/U6 snRNP in the nucleus. The transport signals are retained on the snRNP particles and proteins since existing particles and proteins enter the reformed nucleus after mitosis. .IAbbreviations: snRNP. small nuclear ribonucleoprotein; mRiXA. small nuclear ribonucleic acid: mRNA. messenger RNA: rRNA. ribosomal RNA: pre-mRNA. unspliced mRNA: pre-rRNA, unprocessed rRKA; TMG, trimethylguaiiosinc: m7G. 7-methyl guanosine: MepppG, ymonomethyl guanosinc tripliosphate (GTP): mAb. monoclonal antibody: SLE, systcmic lupus crythcmatosus: MCTD. inixcd coniicctivc tissue discascs; anti-Sni. antibody rccognizing snRNP core protcins: anti-RNP, antibody rccogiiiziiig UI siiRNP protcins.
Introduction Four of the major snRNP particles (U1, u 2 , u5, U4/U6) are stable components of the nucleus that function together in the splicing of all known premRNAs. These snRNP particles (11s) assemble into dynamic spliceosomes (60s) during pre-mRNA splicing reactions through a variety of RNA/protein, RNA/RNA and protein/protein interactions. It is likely that some of the interactions that occur during the initial pre-assembly of snRNP proteins and assembly of snRNP particles in the cytoplasm mimic the interactions that occur during spliceosome assembly in the nucleus. At least thirteen different small ribonucleoprotein particles (snRNP) function to process pre-messenger RNA (pre-mRNA) and pre-ribosomal RNA (prerRNA) in the nucleus of mammalian cell, (for reviews see refs 1-5). The major species of snRNPs involved in the splicing of pre-mRNAs share a common core of basic proteins as well as one (or two) snRNA and several snRNA specific proteins('). Each snRNP is designated by the snRNA(s) (Ul, U2, US, U4/U6) contained in the particle and the snRNA directs the function of its particle in the spliceosome. For example, one function of the U1 snRNP in the splicing reaction of pre-mRNAs is the recognition of the 5' splice site through U1 snRNA base-pair interactions with the 5' splice site, an event enhanced by thc Ul-specific C protein (see refs 1-5). The branch point for lariat formation and the 3' splice site are recognized by U2 and most likely U5 snRNPs, respectively (see refs 2,3). And recently, it has been suggested that U6 snRNA acts as the catalytic component in the spliceosome and is negatively regulated through antisense hybridization with U4 snRNA in the U4/U6 snRNP The minor (or rather less abundant) snRNP particles (U7-U14) (Table 1) are involved in a variety of functions in RNA processing. Three of the characterizcd minor snRNP particles U7, U l l , and U12, which are involved in pre-mRNA processing, contain the same core proteins as the major U1, U2, U4/U6, U5 snRNP particles and additional species of uncharacterized minor snRNAs with TMG caps coimmunoprecipi-
Table 1. snRNA
u1 u2 u3 u4
us
U6 u7 U8 UY u10 u11 u12 u13 U14
Size
Abundance x 10"
5' Cap
Function
165 I89 216 139 117 107 56 139 130 60 131 120 105 87
1.0 0.9 0.2 0.2 0.2 0.4 (0.05
Introduction Four of the major snRNP particles (U1, u 2 , u5, U4/U6) are stable components of the nucleus that function together in the splicing of all known premRNAs. These snRNP particles (11s) assemble into dynamic spliceosomes (60s) during pre-mRNA splicing reactions through a variety of RNA/protein, RNA/RNA and protein/protein interactions. It is likely that some of the interactions that occur during the initial pre-assembly of snRNP proteins and assembly of snRNP particles in the cytoplasm mimic the interactions that occur during spliceosome assembly in the nucleus. At least thirteen different small ribonucleoprotein particles (snRNP) function to process pre-messenger RNA (pre-mRNA) and pre-ribosomal RNA (prerRNA) in the nucleus of mammalian cell, (for reviews see refs 1-5). The major species of snRNPs involved in the splicing of pre-mRNAs share a common core of basic proteins as well as one (or two) snRNA and several snRNA specific proteins('). Each snRNP is designated by the snRNA(s) (Ul, U2, US, U4/U6) contained in the particle and the snRNA directs the function of its particle in the spliceosome. For example, one function of the U1 snRNP in the splicing reaction of pre-mRNAs is the recognition of the 5' splice site through U1 snRNA base-pair interactions with the 5' splice site, an event enhanced by thc Ul-specific C protein (see refs 1-5). The branch point for lariat formation and the 3' splice site are recognized by U2 and most likely U5 snRNPs, respectively (see refs 2,3). And recently, it has been suggested that U6 snRNA acts as the catalytic component in the spliceosome and is negatively regulated through antisense hybridization with U4 snRNA in the U4/U6 snRNP The minor (or rather less abundant) snRNP particles (U7-U14) (Table 1) are involved in a variety of functions in RNA processing. Three of the characterizcd minor snRNP particles U7, U l l , and U12, which are involved in pre-mRNA processing, contain the same core proteins as the major U1, U2, U4/U6, U5 snRNP particles and additional species of uncharacterized minor snRNAs with TMG caps coimmunoprecipi-
Table 1. snRNA
u1 u2 u3 u4
us
U6 u7 U8 UY u10 u11 u12 u13 U14
Size
Abundance x 10"
5' Cap
Function
165 I89 216 139 117 107 56 139 130 60 131 120 105 87
1.0 0.9 0.2 0.2 0.2 0.4 (0.05
