FUNCTIONAL-EFFECTS OF DOMAIN DELETIONS IN A MULTIDOMAIN SERINE-PROTEASE, CLR

The Clr subcomponent of the first component of complement is a complex , multidomain glycoprotein containing five regulatory or binding modul es in addition to the serine protease domain. To reveal the functional role of the N-terminal regulatory domains, two deletion mutants of Cl r were constructed. One mutant comprises the N-terminal half of domain I joined to the second half of the highly homologous domain III, resu lting in one chimeric domain in the N-terminal region, instead of doma ins I-III. In the second mutant most of the N-terminal portion of doma in I was deleted. Both deletion mutants were expressed in the baculovi rus-insect cell expression system with yields typical of wild type Clr . Both mutants maintained the ability of the wild type Clr to dimerize . The folding and secretion of the recombinant proteins was not affect ed by these deletions, and Cl-inhibitor binding was not impaired. The stability of the zymogen was significantly decreased however, indicati ng that the N-terminal region of the Clr molecule contains essential e lements involved in the control of activation of the serine protease m odule. Tetramer formation with Cls in the presence of Ca2+ was abolish ed by both deletions. We suggest that the first domain of Clr is essen tial for tetramer formation, since the deletion of domain I from Clr i mpairs this interaction. Copyright (C) 1996 Elsevier Science Ltd.