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). [6][7] Due to the discovery that the 32-kDa amino terminal fragment of lubricin could stimulate in-vitromegakaryocyte growth, the gene responsible for the expression of lubricin was initially called "megakaryocyte-stimulating factor" (MSF).[8] However, Lubricin, MSF, and SZP are now collectively known as Proteoglycan 4 (hence PRG4 for the gene nomenclature). The evidence that lubricin is actually a proteoglycan is not solid.[9] The expression of lubricin has also been detected and the protein localized in tendon,[10]meniscus,[11]lung, liver, heart, bone,[12]ligament, muscle, and skin.[13] It is present in human plasma, where it binds to neutrophils via L-selectin.[14]
The adhesion of Lubricin's N- (blue) and C- (red) termini to two opposing cartilage surfaces undergoing shear stress (
