Intramolecular chaperones in the intestinal proteins lactase-phlorizin hydrolase and sucrase-isomaltase

Several proteins possess pro-regions or propeptides at the N-terminal end which undergo cleavage during or after maturation of the precursor polypeptide. Cleavage of these propeptides is associated in many cases with biological activation of the final mature form of the protein as in the cases of zymogens, neuropeptides and prohormones. An important function of propeptides is that of an intramolecular chaperone that regulates and affects the folding of the precursor protein. Human small intestinal lactase-phlorizin hydrolase (LPH), an important component of the brush border membrane, comprises a relatively large pro-region (LPHa) which accounts for about 45% of the precursor molecule (pre-pro-LPH, 1927 amino acids) and a brush border mature polypeptide, LPHb. Complex glycosylated mature pro-LPH undergoes an initial intracellular cleavage step that implicates a trypsin-like protease and LPHbinitial. The LPHbinitial molecule is subsequently targeted to the brush-border membrane where a final digestion by luminal trypsin occurs yielding LPHbfinal that exerts its enzymatic function as a ß-galactosidase in the small intestine. Several studies have demonstrated that uncleaved pro-LPH is a transport-competent, apically-sorted and an enzymatically-active molecule. Cleavage of pro-LPH in intestinal cells is therefore neither required for activation of the enzyme, nor is it implicated in the transport and sorting events. Our working hypothesis is that the profragment LPHa acts as an intramolecular chaperone implicated in the folding of the LPHb domain. The temporal and domain-specific requirements for the interaction of LPHa with LPHb and ER resident proteins are being currently analysed. The next step is to determine whether the pro-region of LPH functions also as an intramolecular chaperone in the context of other proteins which also contain pro-regions. This would shed light on a putative general mechanism of function of intramolecular chaperones. Another protein, sucrase-isomaltase (pro-SI), has also common structural features with pro-LPH in that it is composed of two main strongly homologous subunits, sucrase and isomaltase. Unlike pro-LPH, SI is a type II protein and our data have shown that the sucrase subunit may function as an intramolecular chaperone in assisting the folding of the isomaltase domain. In view of the location of sucrase at the C-terminal end of pro-SI sucrase is the first C-terminally located domain that functions as an intramolecular chaperone.