The LSm protein Hfq hexamer torus showing the peptide backbone, with each protein in a different color, representing each beta strand as a ribbon, each alpha helix as a cylinder, and the RNA oligonucleotide as a 300° arc.
LSm proteins are defined by a characteristic three-dimensional structure and their assembly into rings of six or seven individual LSm protein molecules.
The Sm proteins were first discovered as antigens targeted by so called Anti-Sm antibodies in a patient with a form of systemic lupus erythematosus (SLE), a debilitating autoimmune disease. They were named Sm proteins in honor of this patient, Stephanie Smith. Other proteins with very similar structures were subsequently discovered and named LSm proteins. New members of the LSm protein family continue to be identified and reported.
Proteins with similar structures are grouped into a hierarchy of protein families, superfamilies and folds. The LSm protein structure is an example of a small beta sheet folded into a short barrel. Individual LSm proteins assemble into a six or seven member doughnut ring (more properly termed a torus), which usually binds to a small RNA molecule to form a ribonucleoprotein complex. The LSm torus assists the RNA molecule to assume and maintain its proper three dimensional structure. Depending on which LSm proteins and RNA molecule are involved, this ribonucleoprotein complex facilitates a wide variety of RNA processing including degradation, editing, splicing, and regulation.