title: Binding characteristics of the osteoarthritis-associated protein asporin.
abstract: Asporin is an extracellular matrix (ECM) protein that regulates cartilage matrix gene expression and cartilage formation by modulating the transforming growth factor-beta (TGF-beta) signaling pathway. Our previous studies have indicated that asporin binds to TGF-beta1 directly and inhibits TGF-beta1-mediated expression of cartilage matrix genes. However, it is still unknown how asporin interacts with TGF-beta1 and influences its activity. Using competition assays, we determined that amino acids 159-205 of asporin mediate its interaction with TGF-beta1 and effectively repress TGF-beta1-induced cartilage matrix gene expression. Asporin also has a binding ability to type II collagen in vitro, but its binding pattern is different from that of TGF-beta1. In contrast with previous in vivo findings, asporin did not affect the interaction between TGF-beta1 and the TGF-beta type II receptor (TbetaRII) by itself or in the presence of type II collagen in vitro. However, in the presence of heparin/heparan sulfate, asporin inhibits the interaction between TGF-beta and TbetaRII in vitro. These findings suggest that asporin is one of the important cartilage matrix proteins that binds to the ECM and TGF-beta1 and thereby modulates interactions between TGF-beta and its signaling receptors.
authors.affiliation: Laboratory for Bone and Joint Diseases, Center for Genomic Medicine, RIKEN, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
authors.forename: Ikuyo
authors.initials: I
authors.lastname: Kou
authors.forename: Masahiro
authors.initials: M
authors.lastname: Nakajima
authors.forename: Shiro
authors.initials: S
authors.lastname: Ikegawa
mesh_terms: D000818:Animals; D002460:Cell Line; D024043:Collagen Type II / Q000235:genetics / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D016326:Extracellular Matrix Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D000077294:Receptor, Transforming Growth Factor-beta Type II; D018125:Receptors, Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism
chemical_list: C511592:Aspn protein, mouse; D024043:Collagen Type II; D016326:Extracellular Matrix Proteins; D018125:Receptors, Transforming Growth Factor beta; D016212:Transforming Growth Factor beta; D017346:Protein Serine-Threonine Kinases; D000077294:Receptor, Transforming Growth Factor-beta Type II