Proteoglycan 4
Proteoglycan 4 or lubricin is a proteoglycan that in humans is encoded by the PRG4 gene.  It acts as a joint/boundary lubricant.

# Function
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). 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. The expression of lubricin has also been detected and the protein localized in tendon, meniscus, lung, liver, heart, bone, ligament, muscle, and skin. It is present in human plasma, where it binds to neutrophils via L-selectin.

# Structure
The protein encoded by this gene is a approximately 345 kDa  specifically synthesized by chondrocytes located at the surface of articular cartilage, and also by synovial lining cells. The cDNA encodes a protein of 1,404 amino acids (human A isoform) with a somatomedin B homology domain, heparin-binding domains, multiple mucin-like repeats, a hemopexin domain, and an aggregation domain. There are 3 consensus sequences for N-glycosylation and more than 168 sites for O-linked glycosylation
Lubricin is a large glycoprotein that consists of approximately equal proportions of protein and oligosaccharides. The oligosaccharides are O-linked both with and without sialic acid.. Electron microscope measurements show that the lubricin molecule is a partially extended flexible rod and, in solution, occupies a smaller spatial domain than would be expected from structural predictions. The large glycosylated region (i. e mucin domain) of lubricin makes it a water-soluble synovial fluid protein. In synovial fluid it interacts with Galectin-3 that improves its lubricating property. Lubricins unglycosylated regions can interact cartilage proteins. This characteristic may aid in the molecule's boundary lubricating ability.

# Clinical significance
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.
The locus for autosomal recessive camptodactyly-arthropathy-coxa vara-pericarditis syndrome maps to chromosome 1q25-q31 where the PRG4 gene is located.  Cell overgrowth may be primary to the pathogenesis of this protein.
Lubricins role in improving tendon gliding has also been studied. While adding lubricin alone fails to affect the tendon gliding resistance, the addition of cd-gelatin plus lubricin significantly lowered the gliding resistance of the tendons. This research can aid in improving the gliding ability of tendon grafts done clinically.
Extracorporeal shockwave therapy application has been shown to induce an increased lubricin expression in tendons and septa of rat hindlimbs, which might suggest a beneficial lubricanting effect for joints and tissues prone to wear and tear degradation.