Functions LSm

1 functions

1.1 sm ring
1.2 lsm2-8 ring
1.3 sm10/sm11 ring
1.4 lsm1-7 ring
1.5 gemin6 , gemin7
1.6 lsm12-16 , other multi-domain lsm proteins
1.7 archaeal sm rings
1.8 bacterial lsm rings

functions

the various kinds of lsm rings function scaffolds or chaperones rna oligonucleotides, assisting rna assume , maintain proper three-dimensional structure. in cases, allows oligonucleotide rna function catalytically ribozyme. in other cases, facilitates modification or degradation of rna, or assembly, storage, , intracellular transport of ribonucleoprotein complexes.

sm ring

the sm ring found in nucleus of eukaryotes (about 2.5 x 10 copies per proliferating human cell), , has best understood functions. sm ring heteroheptamer. sm-class snrna molecule (in 5 3 direction) enters lumen (doughnut hole) @ sme subunit , proceeds sequentially in clockwise fashion (looking α helix side) inside lumen (doughnut hole) smg, smd3, smb, smd1, smd2 subunits, exiting @ smf subunit. (smb can replaced splice variant smb , smn in neural tissues.) sm ring permanently binds u1, u2, u4 , u5 snrnas form 4 of 5 snrnps constitute major spliceosome. sm ring permanently binds u11, u12 , u4atac snrnas form 4 of 5 snrnps (including u5 snrnp) constitute minor spliceosome. both of these spliceosomes central rna-processing complexes in maturation of messenger rna pre-mrna. sm proteins have been reported part of ribonucleoprotein component of telomerase.

lsm2-8 ring

the 2 lsm2-8 snrnps (u6 , u6atac) have key catalyic function in major , minor spliceosomes. these snrnps not include sm ring, instead use heteroheptameric lsm2-8 ring. lsm rings 20 times less abundant sm rings. order of these 7 lsm proteins in ring not known, based on amino acid sequence homology sm proteins, speculated snrna (in 5 3 direction) may bind first lsm5, , precedes sequentially clockwise lsm7, lsm4, lsm8, lsm2, lsm3, , exiting @ lsm6 subunit. experiments saccharomyces cerevisiae (budding yeast) mutations suggest lsm2-8 ring assists reassociation of u4 , u6 snrnps u4/u6 di-snrnp. (after completion of exon deletion , intron splicing, these 2 snrnps must reassociate spliceosome initiate exon/intron splicing cycle. in role, lsm2-8 ring acts rna chaperone instead of rna scaffold.) lsm2-8 ring forms snrnp u8 small nucleolar rna (snorna) localizes in nucleolus. ribonucleoprotein complex necessary processing ribosomal rna , transfer rna mature forms. lsm2-8 ring reported have role in processing of pre-p rna rnase p rna. in contrast sm ring, lsm2-8 ring not permanently bind snrna , snorna.

sm10/sm11 ring

a second type of sm ring exists lsm10 replaces smd1 , lsm11 replaces smd2. lsm11 2 domain protein c-terminal domain being lsm domain. heteroheptamer ring binds u7 snrna in u7 snrnp. u7 snrnp mediates processing of 3 utr stem-loop of histone mrna in nucleus. sm ring, assembled in cytoplasm onto u7 snrna specialized smn complex.

lsm1-7 ring

a second type of lsm ring lsm1-7 ring, has same structure lsm2-8 ring except lsm1 replaces lsm8. in contrast lsm2-8 ring, lsm1-7 ring localizes in cytoplasm assists in degrading messenger rna in ribonucleoprotein complexes. process controls turnover of messenger rna ribosomal translation of mrna protein responds changes in transcription of dna messenger rna cell.

gemin6 , gemin7

the smn complex (described under biogenesis of snrnps ) composed of smn protein , gemin2-8. 2 of these, gemin 6 , gemin7 have been discovered have lsm structure, , form heterodimer. these may have chaperone function in smn complex assist formation of sm ring on sm-class snrnas. prmt5 complex composed of prmt5, picln, wd45 (mep50). picln helps form sm opened ring on smn complex. smn complex assists in assembly of snrnps sm ring in open conformation on smn complex , sm ring loaded onto snrna smn complex.

lsm12-16 , other multi-domain lsm proteins

the lsm12-16 proteins have been described recently. these two-domain proteins n-terminal lsm domain , c-terminal methyl transferase domain. little known function of these proteins, presumably member of lsm-domain rings interact rna. there evidence lsm12 possibly involved in mrna degradation , lsm13-16 may have roles in regulation of mitosis. large protein of unknown function, ataxin-2, associated neurodegenerative disease spinocerebellar ataxia type 2, has n-terminal lsm domain.

archaeal sm rings

two lsm proteins found in second domain of life, archaea. these sm1 , sm2 proteins (not confused sm1 , sm2 sequence motifs), , identified sm-like archaeal proteins smap1 , smap2 reason. sm1 , sm2 form homoheptamer rings, although homohexamer rings have been observed. sm1 rings similar eukaryote lsm rings in form in absence of rna while sm2 rings similar eukaryote sm rings in require uridine-rich rna formation. have been reported associate rnase p rna, suggesting role in transfer rna processing, function in archaea in process (and possibly processing other rna such ribosomal rna) unknown. 1 of 2 main branches of archaea, crenarchaeotes have third known type of archaeal lsm protein, sm3. two-domain protein n-terminal lsm domain forms homoheptamer ring. nothing known function of lsm protein, presumably interacts with, , helps process, rna in these organisms.

bacterial lsm rings

several lsm proteins have been reported in third domain of life, bacteria. hfq protein forms homohexamer rings, , discovered necessary infection bacteriophage qβ, although not native function of protein in bacteria. not universally present in bacteria, has been found in proteobacteria, firmicutes, spirochaetes, thermotogae, aquificae , 1 species of archaea. (this last instance case of horizontal gene transfer.) hfq pleiotropic variety of interactions, associated translation regulation. these include blocking ribosome binding mrna, marking mrna degradation binding poly-a tails, , association bacterial small regulatory rnas (such dsra rna) control translation binding mrnas. second bacterial lsm protein ylxs (sometimes called yhbc), first identified in soil bacterium bacillus subtilis. two-domain protein n-terminal lsm domain. function unknown, amino acid sequence homologs found in virtually every bacterial genome date, , may essential protein. middle domain of small conductance mechanosensitive channel mscs in escherichia coli forms homoheptameric ring. lsm domain has no apparent rna-binding function, homoheptameric torus part of central channel of membrane protein.