Similarities of integumentary mucin B.1 from Xenopus laevis and prepro-von Willebrand factor at their amino-terminal regions

Frog integumentary mucin B.1 (FIM-B.1) contains various cysteine-rich modules. In the past, a COOH-terminal ''cystine knot'' motif has been found that is similar to von Willebrand factor; this region is generally known to be responsible for dimerization processes. Furthermore, a ''complement control protein'' motif is present as an internal cysteine-rich domain in FIM-B.1. We characterize here the missing 75% toward the NH2 terminus of the FIM B.1 precursor by molecular cloning. Analogous to prepro-von Willebrand factor, four elements with considerable similarity to D-domains are present (i.e. D1-D2-D'-D3). These domains have been described as essential for the multimerization of von Willebrand factor. Thus, the general structure of FIM-B.1 resembles that of the human mucin MUC2 as well as prepro-von Willebrand factor; these three molecules at least seem to share common structural elements allowing similar multimerization mechanisms.