We have defined the structure of the
Osteoclast Functional Antigen (OFA) by immunological and biochemical means. OFA is an abundant
surface antigen in human and animal osteoclasts and has been characterized previously by
monoclonal antibodies 13C2 and 23C6, one of which mimicks the inhibitory activity of
calcitonin on osteoclastic
bone resorption. By the following criteria we show that OFA is a member of the
integrin family of
extracellular matrix receptors and is identical, or at least highly related, to the
vitronectin receptor (VNR) previously isolated from placenta and
melanoma cells. Immunoprecipitation analysis demonstrates that OFA from osteoclasts and a monkey kidney cell line Vero is a heterodimeric molecule of 140 kD (alpha chain) and 85 kD (beta chain) under nonreducing conditions; on reduction at least one low molecular mass (alpha') species (of approximately 30-kD size) is released, resulting in a 120/100-kD dimer. Immunoblots of OFA isolated from osteoclasts and Vero cells and VNR purified from placenta and probed with heterosera to OFA and
monoclonal antibodies to platelet gp111a (VNR beta chain) show immunological cross-reactivity between the alpha chains of OFA and VNR and the use of gp111a as a beta chain by both. OFA from Vero cells binds to an
Arg-Gly-Asp containing
peptide (GRGDSPPK) isolating a heterodimer recognized by anti-OFA
monoclonal antibodies, 13C2 and 23C6. Immunohistochemical analysis showed a similar tissue distribution in humans for the
antigen recognized by anti-OFA
antibodies, a
monoclonal antibody, LM142, raised to
melanoma VNR, polyclonal
antibodies to the placental VNR and a
monoclonal antibody to the presumptive VNR beta chain,
platelet glycoprotein 111a. Finally, NH2 terminal
amino acid sequencing showed that the amino-terminus of the monkey alpha chain was identical in the 12 assigned residues to that of human VNR alpha chain. The beta chain sequence of OFA differed at least 1 (and up to 4) positions from platelet gp111a (VNR beta) in the first 18
amino acids sequenced. These, and other, data provide the first indication of a function for the VNR and suggest that cell-cell and cell-extracellular matrix interactions involving
integrins may play an important role in bone physiology.