Recombinant and tissue-derived mouse BM-40 bind to several collagen types and have increased affinities after proteolytic activation

Abstract.

The calcium-binding extracellular matrix protein BM-40 was obtained as a mouse cDNA product from a stably transfected kidney cell clone. Electrophoresis and N-terminal sequence analysis demonstrated absence of the proteolytic processing previously observed for a mouse tumour-derived BM-40. Yet the two forms of BM-40 were very similar in their CD spectra, their calcium-dependent change in α helix content and their immunological epitopes. In surface plasmon resonance assays, recombinant mouse BM-40 showed distinct binding to the triple-helical domains of collagens I, II, III, IV and V with K_d=1–4 μM but no binding to collagen VI. These interactions were abolished in the presence of EDTA. Tissue-derived mouse BM-40, however, bound collagens I and IV with K_d=0.1–0.2 μM. Activation of collagen binding to give a similar K_d could be achieved for recombinant mouse BM-40 by treatment with the matrix metalloproteinase collagenase-3. The major cleavage site was located in helix C of the extracellular calcium-binding module of BM-40 and other less prominent cleavages occurred close to the N-terminus. The sensitive helix C site was just one residue away from that sensitive to endogenous tissue proteolysis, suggesting that cleavage could be a physiological mechanism to modulate collagen binding.