title: Delta 469 mutation in the type 3 repeat calcium binding domain of cartilage oligomeric matrix protein (COMP) disrupts calcium binding.
abstract: Cartilage oligomeric matrix protein (COMP/TSP5), a large glycoprotein found in the territorial matrix surrounding chondrocytes, is the fifth member of the thrombospondin (TSP) gene family. While the function of COMP is unknown, its importance is underscored by the finding that mutations in the highly conserved type 3 repeat domain causes two skeletal dysplasias. Pseudoachondroplasia (PSACH) and Multiple Epiphyseal Dysplasia, Fairbanks type (EDM1). The type 3 repeats are highly conserved low-affinity Ca(2+)binding domains that are found in all TSP genes. This study was undertaken to determine the effects of mutations on calcium binding and structure of the type 3 repeat domains. Wild-type (WT) and Delta469 recombinant COMP (rCOMP) proteins containing the entire calcium-binding domain were expressed in E. coli and purified. Equilibrium dialysis demonstrated that WT bound 10-12 Ca(2+)ions/molecule while Delta469 bound approximately half the Ca(2+)ions. Circular dichroism (CD) spectrometry had striking spectral changes for the WT in response to increasing concentrations of Ca(2+). These CD spectral changes were cooperative and reversible. In contrast, a large CD spectral change was not observed at any Ca(2+)concentration for Delta469. Moreover, both WT and Delta469 proteins produced similar CD spectral changes when titrated with Zn(2+), Cu(2+)and Ni(2+)indicating that the Delta469 mutation specifically affects only calcium binding. These results suggest that the Delta469 mutation, in the type 3 repeat region, interferes with Ca(2+)binding and that filling of all Ca(2+)binding loops may be critical for correct COMP protein conformation.
authors.affiliation: Department of Pediatrics, University of Texas Houston Medical School, Houston, USA.
authors.forename: J
authors.initials: J
authors.lastname: Hou
authors.forename: J A
authors.initials: JA
authors.lastname: Putkey
authors.forename: J T
authors.initials: JT
authors.lastname: Hecht
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002356:Cartilage / Q000378:metabolism*; D064236:Cartilage Oligomeric Matrix Protein; D002499:Centrifugation, Density Gradient; D002942:Circular Dichroism; D016384:Consensus Sequence; D004252:DNA Mutational Analysis; D004492:Edetic Acid / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D016326:Extracellular Matrix Proteins / Q000235:genetics* / Q000378:metabolism; D017353:Gene Deletion; D005822:Genetic Vectors; D006023:Glycoproteins / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D064235:Matrilin Proteins; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013329:Structure-Activity Relationship
chemical_list: D064236:Cartilage Oligomeric Matrix Protein; D016326:Extracellular Matrix Proteins; D006023:Glycoproteins; D064235:Matrilin Proteins; D011994:Recombinant Proteins; C577553:TSP5 protein, human; D004492:Edetic Acid; D002118:Calcium