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 modules in addition to the serine protease domain. To reveal the functional role of the N-terminal regulatory domains, two deletion mutants of Clr were constructed. One mutant comprises the N-terminal half of domain I joined to the second half of the highly homologous domain III, resulting in one chimeric domain in the N-terminal region, instead of domains I-III. In the second mutant most of the N-terminal portion of domain I was deleted. Both deletion mutants were expressed in the baculovirus-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 affected by these deletions, and Cl-inhibitor binding was not impaired. The stability of the zymogen was significantly decreased however, indicating that the N-terminal region of the Clr molecule contains essential elements involved in the control of activation of the serine protease module. Tetramer formation with Cls in the presence of Ca2+ was abolished by both deletions. We suggest that the first domain of Clr is essential for tetramer formation, since the deletion of domain I from Clr impairs this interaction. Copyright (C) 1996 Elsevier Science Ltd.