InterPro: IPR001163 Like-Sm ribonucleoprotein (LSM) domain

This family is found in Lsm (like-Sm) proteins and in bacterial Lsm-related Hfq proteins. In each case, the domain adopts 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]. In other snRNPs, certain Sm proteins are replaced with different Lsm proteins, such as with U7 snRNPs, in which the D1 and D2 Sm proteins are replaced with U7-specific Lsm10 and Lsm11 proteins, where Lsm11 plays a role in histone U7-specific RNA processing [5]. Lsm proteins are also found in archaebacteria, which do not have any splicing apparatus suggesting a more general role for Lsm proteins.

The pleiotropic translational regulator Hfq (host factor Q) is a bacterial Lsm-like protein, which modulates the structure of numerous RNA molecules by binding preferentially to A/U-rich sequences in RNA [6]. Hfq forms an Lsm-like fold, however, unlike the heptameric Sm proteins, Hfq forms a homo-hexameric ring.