N-Terminal sequences of pig intestinal sucrase-isomaltase and pro-sucrase--isomaltase. Implications for the biosynthesis and membrane insertion of pro-sucrase--isomaltase.

The hog sucrase-isomaltase complex is anchored to the small-intestinal brush border membrane, as in the rabbit, via a hydrophobic segment located in the N-terminal region of the isomaltase subunit. The immediate precursor of the 'final' sucrase-isomaltase (i.e., pro-sucrase-isomaltase as prepared from adult hogs whose pancreas had been disconnected from the duodenum) is an amphiphilic single polypeptide chain of Mr 260000-265000. Its N-terminal sequence is virtually identical with (not merely homologous to) the corresponding region of the isomaltase subunit of 'final' sucrase-isomaltase. This shows that the isomaltase portion of pro-sucrase-isomaltase is the N-terminal 'half' of the precursor polypeptide chain. Thus the succession of domains in pro-sucrase-isomaltase and its mode of anchoring in the membrane could be deduced. On this basis a likely mechanism of biosynthesis and insertion is proposed.