Cloning of rat Muc5AC mucin gene: comparison of its structure and tissue distribution to that of human and mouse homologues.

The human mucin gene MUC5AC codes for a large mucin which has tandem repeat units and cysteine rich regions characteristic of several members of this class of glycoproteins. Human epithelia expressing the mucin include that of stomach, bronchus/trachea, endocervix and conjunctiva. We report here a 3.8 kb partial sequence of a rat homologue for the human MUC5AC gene and compare its tandem repeat sequence and cysteine rich domains to those of the human and mouse gene. Rat and mouse have the same number of amino acids (16) in their Muc5AC tandem repeat units and share 69% sequence similarity, whereas human MUC5AC has only 8 amino acids in its tandem repeat. In rat, the tandem repeat domain is flanked at its 3' end by a non-repeat region coding for 1142 amino acids. Four cysteine rich subdomains were identified in this region; one of these has 64% similarity to a corresponding region in human MUC5AC and 80% similarity to a mouse MUC5AC cysteine rich region. Southern blot analysis revealed cross hybridization of a probe for the rat cysteine rich region, to human, mouse, rabbit, and porcine genomic DNA; the rat tandem repeat probe hybridized with mouse and rabbit only. Unlike humans, rat expressed MUC5AC message detectable by Northern blot and in situ hybridization only in stomach epithelium and conjunctival goblet cells.