EFFECT OF LACTOPEROXIDASE-CATALYZED IODINATION ON THE CA2+-DEPENDENT INTERACTIONS OF HUMAN C1S - LOCATION OF THE IODINATION SITES

C1BARs, one of the two serine proteases of C1BAR, the first component of complement, has the ability to mediate heterologous (C1BARr-C1BARs) as well as homologous (C1BARs-C1BARs) Ca2+-dependent interactions, both involving the NH2-terminal alpha-region of its A chain. Lactoperoxidase-catalyzed iodination of C1BARs in its monomeric form was found to abolish its ability to form Ca2+-dependent homodimers, without impairing its ability to mediate C1BARr-C1BARs heteroassociation. C1BARs iodinated in its dimeric form, in contrast, fully retained the ability to self-associate. With a view to identify the tyrosine residues iodinated in each case, C1BARs was radioiodinated in its monomeric and dimeric forms, and comparative tryptic mapping was performed on the resulting I-125-labeled A chains. Most of the tyrosine residues either were not iodinated or were equivalently iodinated in both monomeric and dimeric C1BARs. In contrast, Tyr-33 and Tyr-38 were iodinated in the monomer but not in the dimer. Conversely, Tyr-52 and Tyr-147 were iodinated only in the dimer. These results provide further evidence that the structural determinants of C1BARs required for Ca2+ binding and Ca2+-dependent protein-protein interactions are contributed by both the NH2-terminal motif I (positions 1-1 10) and the epidermal growth factor like motif II (positions 111-159) of the alpha-region. On the basis of available information, tentative models of the C1BARs-C1BARs and C1BARr-C1BARs Ca2+-dependent interactions are proposed.