SOLUTION STRUCTURE OF THE EPIDERMAL GROWTH-FACTOR (ECF)-LIKE MODULE OF HUMAN-COMPLEMENT PROTEASE C1R, AN ATYPICAL MEMBER OF THE EGF FAMILY

The calcium-dependent interaction between C1r and C1s, the two homolog ous serine proteases of the first component of human complement C1, is mediated by their N-terminal regions. The latter comprise an epiderma l growth factor (EGF)-like module exhibiting the consensus sequence ch aracteristic of Ca2+-binding EGF modules, surrounded by two CUB module s. Due to its Ca2+ binding ability, the C1r EGF-like module (C1r-EGF) is supposed to participate in the C1r-C1s interaction. An additional i nteresting feature of C1r-EGF is the unusually large loop connecting t he first two conserved cysteine residues. The solution structure of sy nthetic C1r-EGF (residues 123-175) has been determined using nuclear m agnetic resonance and combined simulated annealing-restrained molecula r dynamics calculations. The resulting family of 19 structures is char acterized by a well-ordered C-terminal part (residues Cys144-Ala174) w ith a backbone rmsd of 0.7 Angstrom and a disordered N-terminal, inclu ding the large loop between the first two cysteines (Cys129 and Cys144 ). This loop is known to be surface exposed and may be expected to par ticipate in domain-domain or protein-protein interactions. In its C-te rminal part, C1r-EGF possesses the characteristic EGF fold with a majo r and a minor beta-sheet. The latter comprises a beta-bulge, and compa rison with other EGF-like modules reveals the existence of two distinc t structural and sequential motifs in the bulged part. Additional expe riments in the presence of 80 mM Ca2+ did not show significant structu ral variation of C1r-EGF, in keeping with previous observations on blo od-clotting factors IX and X.