Human fibroblast collagenase-alpha-macroglobulin interactions. Localization of cleavage sites in the bait regions of five mammalian alpha-macroglobulins

The interaction between human fibroblast collagenase and five mammalian alpha-macroglobulins (human alpha 2-macroglobulin and pregnancy zone protein, rat alpha 1- and alpha 2-macroglobulin, and rat alpha 1-inhibitor 3) differing in primary and quaternary structure has been investigated. Complex formation with each of these alpha-macroglobulins follows the course identified for many other proteinases, i.e. specific limited proteolysis in their bait regions inducing a set of conformational changes resulting in activation of the internal beta-cysteinyl-gamma-glutamyl thiol esters and covalent complex formation. At collagenase: alpha-macroglobulin molar ratios of less than 1:1 3.2-3.6 mol of SH groups appear for 1 mol of collagenase bound to human and rat alpha 2-macroglobulin and to rat alpha 1-macroglobulin. For these alpha-macroglobulins it can be estimated that the overall rate constant of complex formation is greater than 1.10(6) M-1 s-1 while it is much lower for human pregnancy zone protein and rat alpha 1-inhibitor 3. More than 95% of the complexed collagenase is covalently bound, and sodium dodecyl sulfate gel electrophoresis shows the typical pattern of bands corresponding to reaction products of very high apparent molecular weight. The same pattern is also seen in the covalent (greater than 98%) complex very slowly formed from Clostridium histolyticum collagenase and human alpha 2-macroglobulin. The identification of the sites of specific limited proteolysis in the bait regions of the five alpha-macroglobulins shows that cleavage may take place in sequences that are not related to those identified earlier in the collagens. These results greatly expand the repertoire of sequences known to be cleaved by fibroblast collagenase and suggest that this proteinase has a primary substrate specificity resembling that of the microbial proteinase thermolysin, as it preferentially cleaves at the NH2-terminal side of large hydrophobic residues. In addition, the results highlight the unique structure of the flexible alpha-macroglobulin bait region in that it can accommodate a conformation required by the highly restrictive fibroblasts collagenase. It is suggested that alpha-macroglobulins may play an important role in locally controlling the activity of collagenases and perhaps other proteinases of the extracellular matrix.