The 175 antigen expressed on myeloid and erythroid cells during differentiation is associated with serine protease activity

Monoclonal antibody 175 recognizes a cell-surface antigen on more than 80% of nucleated ovine bone marrow cells (BMC). The distribution is unusual, as the majority of differentiated myeloid and erythroid cells express the antigen (175 antigen), whereas mast cells, basophils, and the majority of lymphocytes do not. The level of 175 antigen expression has been shown to increase as BMC differentiate during hematopoiesis. Previous attempts to identify the 175 antigen have been unsuccessful. In this study, the 175 antigen was affinity-purified and shown to contain serine protease activity. Immunoblot analysis following sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) of bone marrow cell lysates run under reducing or nonreducing conditions showed two closely adjacent protein bands (a doublet) of 28 to 30 kD molecular weight. N-linked deglycosylation showed that the 30-kD band was a glycosylated form of the 28-kD protein. Both protein bands shared the same N-terminal amino acid sequence over 20 residues, with high homology with serine proteases. Affinity-purified 175 antigen was proteolytic in substrate gels, the activity being inhibited by the 175 monoclonal antibody (Mab) and the serine protease inhibitor phenylmethylsulfonyl fluoride (PMSF), but not by metallo, thiol, or acid protease-specific inhibitors. The 175 antigen is therefore part of a growing family of cell-surface proteases associated with hematopoietic cell differentiation.