Lubricin is a large, water soluble glycoprotein encoded by the PRG4 gene. It has a molecular weight of 206,000 Daltons and consists of approximately equal proportions of protein and glycosaminoglycans. Electron microscope measurements show that the lubricin molecule is a partially extended flexible rod and, in solution, occupies a smaller spacial domain than would be expected from structural predictions. This characteristic may aid in the molecule's boundary lubricating ability.
Lubricin is present in synovial fluid and on the surface (superficial layer) of articular cartilage and therefore plays an important role in joint lubrication and synovial homeostasis. When first isolated, cartilage lubricin was called "superficial zone protein" (SZP). Human synovial fibroblasts have been shown to produce lubricin from the same gene responsible for megakaryocyte stimulating factor (MSF). Lubricin, MSF and SZP are now collectively known as Proteoglycan 4 (hence PRG4 for the gene nomenclature). The expression of lubricin has also been detected and the protein localised in tendon, meniscus, lung, liver, heart, bone, ligament, muscle and skin.
Lubricin, as MSF, was detected in the urine of patients undergoing bone marrow transplantation during a period of acute thrombocytopenia. Depletion of lubricin function has also been associated with camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP), an arthritis-like autosomal recessive disorder.
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