Novel Sm-like proteins with long C-terminal tails and associated methyltransferases

Sm and Sm-like proteins of the Lsm (like Sm) domain family are generally involved in essential RNA-processing tasks. While recent research has focused on the function and structure of small family members, little is known about Lsm domain proteins carrying additional domains. Using an integrative bioinformatics approach, we discovered five novel groups of Lsm domain proteins (Lsm12-16) with long C-terminal tails and investigated their functions. All of them are evolutionarily conserved in eukaryotes with an N-terminal Lsm domain to bind nucleic acids followed by as yet uncharacterized C-terminal domains and sequence motifs. Based on known yeast interaction partners, Lsm12-16 may play important roles in RNA metabolism. Particularly, Lsm12 is possibly involved in mRNA degradation or tRNA splicing, and Lsm13-16 in the regulation of the mitotic G2/M phase. Lsm16 proteins have an additional C-terminal YjeF_N domain of as yet unknown function. The identification of an additional methyltransferase domain at the C-terminus of one of the Lsm12 proteins also led to the recognition of three new groups of methyltransferases, presumably dependent on S-adenosyl-l-methionine. Further computational analyses revealed that some methyltransferases contain putative RNA-binding helix-turn-helix domains and zinc fingers.