This family is found in Lsm (like-Sm) proteins, which have a core structure consisting of an open beta-barrel with an SH3-like topology.

Lsm (like-Sm) proteins have diverse functions, and are thought to be important modulators of RNA biogenesis and function [1, 2]. The Sm proteins form part of specific small nuclear ribonucleoproteins (snRNPs) that are involved in the processing of pre-mRNAs to mature mRNAs, and are a major component of the eukaryotic spliceosome. Most snRNPs consist of seven Sm proteins (B/B', D1, D2, D3, E, F and G) arranged in a ring on a uridine-rich sequence (Sm site), plus a small nuclear RNA (snRNA) (either U1, U2, U5 or U4/6) [3]. All Sm proteins contain a common sequence motif in two segments, Sm1 and Sm2, separated by a short variable linker [4]. Other snRNPs, such as U7 snRNP, can contain different Lsm proteins.

Lsm proteins are also found in archaebacteria, which do not have any splicing apparatus, suggesting a more general role for Lsm proteins. Archaeal Lsm proteins have been shown to bind to small RNAs and are probably involved in many cellular processes [5]. Archaeal Lsm proteins are likely to represent the ancestral Lsm domain [6].

1. Functions of Lsm proteins in mRNA degradation and splicing. Curr. Opin. Cell Biol. 12, 346-50
2. Lsm Proteins are required for normal processing and stability of ribosomal RNAs. J. Biol. Chem. 278, 2147-56
3. Why do cells need an assembly machine for RNA-protein complexes? Trends Cell Biol. 14, 226-32
4. snRNP Sm proteins share two evolutionarily conserved sequence motifs which are involved in Sm protein-protein interactions. EMBO J. 14, 2076-88
5. The archaeal Lsm protein binds to small RNAs. J. Biol. Chem. 285, 34429-38
6. Crystal structure of a heptameric Sm-like protein complex from archaea: implications for the structure and evolution of snRNPs. J. Mol. Biol. 309, 915-23

This family is found in Lsm (like-Sm) proteins, which have a core structure consisting of an open
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