Molecular mapping of statherin- and histatin-binding domains in human salivary mucin MG1 (MUC5B) by the yeast two-hybrid system

MG1 is a high-molecular-weight mucin secreted by mucous acinar cells in hum an submandibular and sublingual glands. We have recently shown that the tra cheobronchial mucin MUC5B is a major component of MG1. MUC5B is organized i nto cysteine-rich N- and C-terminal regions that flank a central tandem-rep eat region containing cysteine-rich subdomains and imperfect 29-residue tan dem repeats. In earlier work, we have shown that this mucin selectively for ms heterotypic complexes with amylase, proline-rich proteins, statherin, an d histatins in salivary secretions, and the aim of this study was to identi fy specific binding domains within MUC5B using the yeast two-hybrid system. Interactions of cysteine-rich domains in the tandem-repeat region (Cys1-Cy s4) and C-terminal region (Cys8a, Cys8b, Cys8c) of MUC5B with statherin and histatins were investigated. These studies indicated that histatin I selec tively bound to Cys1 and Cys2, whereas statherin and histatin 1, 3, and 5 s electively bound to Cys8a. Analysis of the primary sequences of the identif ied binding domains suggests that these domains most probably can fold into globular-like structures in the native mucin. A ProDom blast search reveal ed that sequences in Cys1, Cys2, and Cys8a exhibit similarity to domains in evolutionarily diverse extracellular proteins known to participate in a wi de variety of protein-protein interactions.