In molecular biology, LSm proteins are a family of RNA-binding proteins found in virtually every cellular organism. LSm is a contraction of 'like Sm', because the first identified members of the LSm protein family were the Sm proteins. LSm proteins are defined by a characteristic three-dimensional structure and their assembly into rings of six or seven individual LSm protein molecules, and play a large number of various roles in mRNA processing and regulation.

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 Stephanie Smith, a patient who suffered from SLE. 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.

Alternate terms for LSm family are LSm fold and Sm-like fold, and alternate capitalization styles such as lsm, LSM, and Lsm are common and equally acceptable.

The story of the discovery of the first LSm proteins begins with a young woman, Stephanie Smith, who was diagnosed in 1959 with systemic lupus erythematosus (SLE), eventually succumbing to complications of the disease in 1969 at the age of 22. During this period, she was treated at New York's Rockefeller University Hospital, under the care of Dr. Henry Kunkel and Dr. Eng Tan. As those with an autoimmune disease, SLE patients produce antibodies to antigens in their cells' nuclei, most frequently to their own DNA. However, Dr. Kunkel and Dr. Tan found in 1966 that Ms. Smith produced antibodies to a set of nuclear proteins, which they named the 'smith antigen' (Sm Ag). About 30% of SLE patients produce antibodies to these proteins, as opposed to double stranded DNA. This discovery improved diagnostic testing for SLE, but the nature and function of this antigen was unknown.

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