Sm-like ribonucleoproteins superfamily

This domain is found as the core structure in Lsm (like-Sm) proteins and bacterial Lsm-related Hfq proteins, and as the middle domain of the mechanosensitive channel protein MscS. In each case, the domain adopts a core structure consisting of an open beta-barrel with an SH3-like topology.

Lsm proteins have diverse functions, and are thought to be important modulators of RNA biogenesis and function [10801455, 12438310]. 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. These snRNPs consist of seven Sm proteins (B/B¿, D1, D2, D3, E, F and G), plus a small nuclear RNA (snRNA) (either U1, U2, U5 or U4/6) [15130578]. 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.

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 [12093755]. Hfq forms an Lsm-like fold, however, unlike the heptameric Sm proteins, Hfq forms a homo-hexameric ring.

The middle domain of the mechanosensitive channel of small conductance protein (MscS or YggB) structurally resembles an Lsm protein. MscS is a mechanosensitive channel present in the membrane of bacteria, archaea and eukarya that responds both to stretching of the cell membrane and to membrane depolarisation [12446901]. MscS folds as a homo-heptamer with a cylindrical shape, and can be divided into transmembrane and extramembrane regions: an N-terminal periplasmic region, a transmembrane region, and a C-terminal cytoplasmic region. The C-terminal cytoplasmic region can be further divided into middle and C-terminal domains, which together create a framework that connects to the cytoplasm through distinct openings. The middle domain exhibits an Lsm-like structure, consisting of five beta-strands that pack together with those of other subunits to form a barrel-like sheet extending around the entire protein.