Molecular cloning of a cDNA coding for a region of an apoprotein from the 'insoluble' mucin complex of rat small intestine.

The major part of rat small intestinal mucins occurs as an 'insoluble' glycoprotein complex unextractable in 6 M guanidinium chloride unless disulfide bonds are cleaved. One of the trypsin-resistant high-glycosylated domains of this complex (glycopeptide A) was recently isolated. We have now deglycosylated it with HF, injected it into rabbits and the obtained antiserum was used for expression cloning providing a cDNA clone (VR-1A). This clone contained an open reading frame of 235 amino acids composed of two regions. The deduced N-terminal 53 amino acids included seven Cys residues and only one Ser, followed by a region of 182 residues with 64% Ser and Thr but devoid of Cys residues. Analysis of mRNA revealed a transcript of about 12 kb, identical in size to a band labelled with a probe based on the rat mucin-like protein (MLP/Muc2) cDNA. Pulsed-field gel electrophoresis of genomic rat DNA showed identical bands (380 and 500 kb) when blots were sequentially probed with the MLP/Muc2 probe and VR-1A. A panel of mouse x rat hybrids was used to localize the gene corresponding to both VR-1A and Muc2 to rat chromosome 1. The results strongly suggest that the 'insoluble' mucin complex of the rat small intestine is encoded by the Muc2 gene.