proteoglycan-4 (lubricin, megakaryocyte-stimulating factor, superficial zone proteoglycan, PRG4, MSF, SZP)
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Function
- role in boundary lubrication within articulating joints
- prevents protein deposition onto cartilage from synovial fluid
- controls adhesion-dependent synovial growth
- inhibits adhesion of synovial cells to cartilage surface
- isoform F serves role as a growth factor acting on primitive cells of both hematopoietic & endothelial cell lineages
Structure
- homodimer, disulfide-linked
- N-glycosylated, O-glycosylated
- contains glycosaminoglycan chondroitin sulfate & keratan sulfate
- disulfide bond between Cys-1146 & Cys-1403 is essential for protein cleavage
- different forms of various molecular weight
- contains 2 hemopexin-like domains contains 2 SMB domains (somatomedin B domains)
Compartment
Alternative splicing
named isoforms=6
Expression
- expressed in synovial tissue, cartilage, liver > heart, lung
- isoform B is expressed in kidney, lung, liver, heart, brain
- isoforms C & D are widely expressed
Pathology
- defects in PRG4 are the cause of camptodactyly-arthropathy-coxa vara-pericarditis syndrome (Jacobs syndrome)