Biosynthesis and assembly of the largest and major intrinsic polypeptide of the small intestinal brush borders.
The sucrase-isomaltase complex (SI) of the small intestinal brush border membrane accounts for approximately 9-10% of the intrinsic protein. The isomaltase subunit alone interacts with the membrane directly, via a highly hydrophobic segment at its N-terminal region. This segment has a helical conformation for more than 85% and crosses the membrane twice, the N-terminus being located at the outer, luminal side of the membrane. The sucrase subunit is attached to the membrane solely via its interactions with the isomaltase subunit. The sucrase-isomaltase complex is synthesized as a single, very long (Mr approximately 260 000) polypeptide chain (pro-SI, carrying the two sites of sucrase and isomaltase in an already enzymically active form), with the isomaltase portion corresponding to the N-terminal part of pro-SI. Pro-SI is processed into 'final' SI by pancreatic proteases. Recently the cell-free translation of pro-SI has been achieved in vitro. From a detailed knowledge of the anchoring of SI (and pro-SI) in the membrane it has been possible to suggest one particular mechanism as the most likely for the synthesis, insertion and assembly of pro-SI.
D000595:Amino Acid Sequence; D000818:Animals; D002462:Cell Membrane / Q000201:enzymology*; D002942:Circular Dichroism; D007421:Intestine, Small / Q000201:enzymology*; D007668:Kidney / Q000201:enzymology; D008871:Microvilli / Q000201:enzymology*; D009097:Multienzyme Complexes / Q000096:biosynthesis*; D051381:Rats; D013394:Sucrase-Isomaltase Complex / Q000096:biosynthesis* / Q000378:metabolism; D013552:Swine
D009097:Multienzyme Complexes; D013394:Sucrase-Isomaltase Complex