title: Disease-causing mutations in cartilage oligomeric matrix protein cause an unstructured Ca2+ binding domain.
abstract: Chondrocytes from pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1) patients display an enlarged rough endoplasmic reticulum that accumulates extracellular matrix proteins, including cartilage oligomeric matrix protein (COMP). Mutations that cause PSACH and EDM1 are restricted to a 27-kDa Ca(2+) binding domain (type 3 repeat). This domain has 13 Ca(2+)-binding loops with a consensus sequence that conforms to Ca(2+)-binding loops found in EF hands. Most disease-causing mutations are found in the 11-kDa C-terminal region of this domain. We expressed recombinant native and mutant forms of the type 3 repeat domain (T3) and its 11-kDa C-terminal region (T3-Cterm). T3 and T3-Cterm bind approximately 13 and 8 mol of Ca(2+)/mol of protein, respectively. CD, one-dimensional proton, and two-dimensional (1)H-(15)N HSQC spectra of Ca(2+)-bound T3-Cterm indicate a distinct conformation that has little helical secondary structure, despite the presence of 13 EF hand Ca(2+)-binding loops. This conformation is also formed within the context of the intact T3. 19 cross-peaks found between 9.0 and 11.4 ppm are consistent with the presence of strong hydrogen bonding patterns, such as those in beta-sheets. Removal of Ca(2+) leads to an apparent loss of structure as evidenced by decreased dispersion and loss of all down field resonances. Deletion of Asp-470 (a mutation found in 22% of all PSACH and EDM1 patients) decreased the Ca(2+)-binding capacity of both T3 and T3-Cterm by about 3 mol of Ca(2+)/mol of protein. Two-dimensional (1)H-(15)N HSQC spectra of mutated T3-Cterm showed little evidence of defined structure in the presence or absence of Ca(2+). The data demonstrate that Ca(2+) is required to nucleate folding and to maintain defined structure. Mutation results in a partial loss of Ca(2+)-binding capacity and prevents Ca(2+)-dependent folding. Persistence of an unstructured state of the mutated Ca(2+) binding domain in COMP is the structural basis for retention of COMP in the rough endoplasmic reticulum of differentiated PSACH and EDM1 chondrocytes.
authors.affiliation: Department of Biochemistry, Structural Biology Research Center, University of Texas, Houston Medical School, Houston, Texas 77030, USA.
authors.forename: Quinn
authors.initials: Q
authors.lastname: Kleerekoper
authors.forename: Jacqueline T
authors.initials: JT
authors.lastname: Hecht
authors.forename: John A
authors.initials: JA
authors.lastname: Putkey
mesh_terms: D000130:Achondroplasia / Q000378:metabolism*; D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D064236:Cartilage Oligomeric Matrix Protein; D002454:Cell Differentiation; D002942:Circular Dichroism; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D016326:Extracellular Matrix Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D006023:Glycoproteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D064235:Matrilin Proteins; D008954:Models, Biological; D008969:Molecular Sequence Data; D009154:Mutation; D010009:Osteochondrodysplasias / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D064236:Cartilage Oligomeric Matrix Protein; D018076:DNA, Complementary; D016326:Extracellular Matrix Proteins; D006023:Glycoproteins; D064235:Matrilin Proteins; D011994:Recombinant Proteins; C577553:TSP5 protein, human; D002118:Calcium