Functions of the structural domains of cartilage superficial zone proteoglycan/proteoglycan 4 (SZP/PRG4).

Proteoglycan 4 (PRG4) is a mucinous proteoglycan with an apparent molecular weight of -345 kDa. It has been detected in a variety of tissues including cartilage, tendon, bone heart and liver, and is also known as cartilage superficial zone protein (SZP), lubricin, megakaryocyte stimulating factor (MSF) precursor protein and camptodactyly-arthropathy-coxa vara pericarditis (CACP) protein. PRG4 has been shown to reduce friction in joints, and immunological studies have located PRG4 on the surface of articular cartilage, in synovial fluid, on the surface of meniscus and on the surface of mature compressed tendon. PRG4 is involved in the congenital joint pathology known as CACP. Analyses of the PRG4 sequence reveal a propensity for a diverse number of functions including lubrication, matrix-binding, self-aggregation, cytoprotection and cell proliferation. This study set out to investigate the potential functions of the N- (exons 2-5) and C-terminal (exons 7-12) structural domains of human and bovine PRG4, facilitated by recombinant protein expression. Preliminary results from solid-phase binding assays showed an interaction between the N- terminal domain of PRG4 and plasminogen activator inhibitor-1 (PAI-1), type II collagen and a 70kDa fibronectin fragment. Immunoprecipitaion experiments demonstrated a potential interaction between the PRG4 C-terminal domain and fibronectin fragments. Both the N- and C- terminal contain functional heparin binding sites. The C-terminal domain was shown to interact with superficial zone chondrocytes, an interaction that was perturbed by heparin. The study also used purified full-length PRG4 from three sources: human recombinant, bovine articular cartilage and bovine tendon. These PRG4 species bound to heparin and displayed similar glycosylation profiles. This study also shows that PRG4 is susceptible to degradation by a number of matrix proteinases including elastase, plasmin, matrix metalloproteinase-7 and cathepsin B. Collectively, the data presented in this thesis provides novel information concerning the biochemistry, susceptibility to digestion and functional capabilities of PRG4