title: Cartilage oligomeric matrix protein forms protein complexes with synovial lubricin via non-covalent and covalent interactions.
abstract: Understanding the cartilage surface structure, lost in arthritic disease, is essential for developing strategies to effectively restore it. Given that adherence of the lubricating protein, lubricin, to the cartilage surface is critical for boundary lubrication, an interaction with cartilage oligomeric matrix protein (COMP) was investigated. COMP, an abundant cartilage protein, is known to be important for matrix formation.
  
  
  
  Synovial fluid (SF) from arthritic patients was used to detect possible COMP-lubricin complexes by immunological methods. Recombinant (RC) COMP and lubricin fragments were expressed to characterize this bonding and mass spectrometry employed to specifically identify the cysteines involved in inter-protein disulfide bonds.
  
  
  
  COMP-lubricin complexes were identified in the SF of arthritic patients by Western blot, co-immunoprecipitation and sandwich ELISA. RC fragment solid-phase binding assays showed that the C-terminal (amino acids (AA) 518-757) of COMP bound non-covalently to the N-terminal of lubricin (AA 105-202). Mass spectrometry determined that although cysteines throughout COMP were involved in binding with lubricin, the cysteines in lubricin were primarily focused to an N-terminal region (AA 64-86). The close proximity of the non-covalent and disulfide binding domains on lubricin suggest a two-step mechanism to strongly bind lubricin to COMP.
  
  
  
  These data demonstrate that lubricin forms a complex network with COMP involving both non-covalent and covalent bonds. This complex between lubricin and the cartilage protein COMP can be identified in the SF of patients with arthritis conditions including osteoarthritis (OA) and rheumatoid arthritis (RA).
authors.affiliation: Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: sarah.flowers@medkem.gu.se.
authors.forename: S A
authors.initials: SA
authors.lastname: Flowers
authors.affiliation: Department of Clinical Sciences Lund, Division of Rheumatology and Molecular Skeletal Biology, Lund University, Lund, Sweden. Electronic address: sebastian.kalamajski@imbim.uu.se.
authors.forename: S
authors.initials: S
authors.lastname: Kalamajski
authors.affiliation: Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: l.z.ali@dundee.ac.uk.
authors.forename: L
authors.initials: L
authors.lastname: Ali
authors.affiliation: Department of Rheumatology and Inflammation Research, Institute of Medicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: lena.i.bjorkman@vgregion.se.
authors.forename: L I
authors.initials: LI
authors.lastname: Björkman
authors.affiliation: Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: jaison_505@yahoo.co.in.
authors.forename: J R
authors.initials: JR
authors.lastname: Raj
authors.affiliation: Department of Clinical Sciences Lund, Division of Rheumatology and Molecular Skeletal Biology, Lund University, Lund, Sweden. Electronic address: anders.aspberg@med.lu.se.
authors.forename: A
authors.initials: A
authors.lastname: Aspberg
authors.affiliation: Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: niclas.karlsson@medkem.gu.se.
authors.forename: N G
authors.initials: NG
authors.lastname: Karlsson
authors.affiliation: Department of Medical Biochemistry and Cell Biology, Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Electronic address: chunsheng.jin@medkem.gu.se.
authors.forename: C
authors.initials: C
authors.lastname: Jin
mesh_terms: D001168:Arthritis / Q000378:metabolism*; D001172:Arthritis, Rheumatoid / Q000378:metabolism; D001665:Binding Sites; D064236:Cartilage Oligomeric Matrix Protein / Q000378:metabolism*; D002358:Cartilage, Articular / Q000378:metabolism*; D003545:Cysteine / Q000378:metabolism; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D010003:Osteoarthritis / Q000378:metabolism; D011485:Protein Binding; D025241:Spondylarthritis / Q000378:metabolism; D013582:Synovial Fluid / Q000378:metabolism*; D000099158:Proteoglycan 4
chemical_list: D064236:Cartilage Oligomeric Matrix Protein; D003545:Cysteine; D006023:Glycoproteins; D000099158:Proteoglycan 4; C577553:TSP5 protein, human
keywords: Boundary lubrication; Cartilage degradation; Cartilage oligomeric matrix protein; Lubricin; O-linked glycoproteins; Proteomics