Osteopontin: its transglutaminase-catalyzed posttranslational modifications and cross-linking to fibronectin

Osteopontin (OP) is a component of extracellular, bone, and urinary stone matrices, but the mechanism by which it is stably incorporated into such matrices remains unknown. By SDS-PAGE analysis of [125I]OP, treated with a catalytic amount of TG, we first demonstrate both intra- and intermolecular covalent cross-linking of OP. Most importantly, the analysis of the products generated from reactions containing OP, Fn, and TG by SDS-PAGE, autoradiography, and Western blotting using either OP or Fn antibody, and quantitation of TG-catalyzed epsilon-(gamma-glutamyl)lysine isopeptide formation between OP and Fn demonstrate, for the first time, covalent cross-linking between these two proteins. Similar reactions in the presence of polyamine substrates of TG show OP-Fn intermolecular cross-linking via N,N-bis-(gamma-glutamyl)polyamine formation. Finally, immunoprecipitation of 125I-labeled NRK cell surface proteins with anti-OP and anti-Fn antibodies, SDS-PAGE analysis, and autoradiography provides critical evidence for nonreducible OP-Fn cross-linking in vivo. These results clearly suggest that TG-mediated cross-linking between OP and Fn represents one of the most likely mechanisms by which OP becomes covalently linked to bone matrix, urinary stone matrix, and to ECM.