EVOLUTIONARY CONSERVED RIGID MODULE-DOMAIN INTERACTIONS CAN BE DETECTED AT THE SEQUENCE LEVEL - THE EXAMPLES OF COMPLEMENT AND BLOOD-COAGULATION PROTEASES

Several extracellular modular proteins, including proteases of the com plement and blood coagulation cascades, are shown here to exhibit cons erved sequence patterns specific for a particular module-domain associ ation. This was detected by comparative analysis of sequence variabili ty in different multiple sequence alignments, which provides a new too l to investigate the evolution of modular proteins. A first example de als with the proteins featuring a common complement control protein (C CP) module-serine protease (SP) domain pattern at their C-terminal end , defined here as the CCP-SP sub-family. These proteins include the; c omplement proteases C1r, C1s and MASPs, the Limulus clotting factor C, and the proteins of the haptoglobin family. A second example deals wi th blood coagulation factors VII, IX and X and protein C, all featurin g a common epidermal growth factor (EGF)-SPC-terminal assembly. Highly specific motifs are found at the connection between the CCP or EGF mo dule and the activation peptide of the SP domain: [P/A]-x-C-x-[P/A]-[I /V]-C-G-x-[P/S/K] in the case of the CCP-SP proteins, and C-x-[P/S]-x- x-x-[Y/F]-P-C-G in the case of the EGF-SP proteins. Each motif is stri ctly conserved in the whole sub-family and it is detected in no more t han one other known protein sequence. Strikingly, most of the conserve d residues specific to each sub-family appear to be clustered at the i nterface between the SP domain and the CCP or EGF module. We propose t hat a rigid module-domain interaction occurs in these proteins and has been conserved through evolution. The functional implications of thes e assemblies, underlined by such evolutionary constraints, are discuss ed. (C) 1998 Academic Press.