The pro region of human intestinal lactase-phlorizin hydrolase.

ABSTRACT

Human small intestinal lactase-phlorizin hydrolase (LPH) is synthesized as a single-chain polypeptide precursor, prepro-LPH, that undergoes two sequential cleavage steps: the first in the endoplasmic reticulum to pro-LPH (215-kDa) and the second, following terminal glycosylation in the Golgi apparatus, to mature 160-kDa LPH (denoted LPH beta). The LPH beta molecule is subsequently targetted to the brush-border membrane. Characterization of the N-terminal profragment (denoted LPH alpha) of pro-LPH using an epitope-specific, anti-peptide polyclonal antibody reveals that LPH alpha (i) has an apparent molecular weight of approximately 100,000, (ii) is not associated with LPH beta after cleavage of pro-LPH has occurred, and (iii) is not transported to the cell surface or secreted into the extracellular medium. In biosynthetic labeling experiments, a clear precursor/product relationship could be demonstrated between pro-LPH and the LPH alpha and LPH beta polypeptides. Further, LPH alpha has a significantly shorter half-life than LPH beta. LPH alpha is neither N- nor O-glycosylated, despite the presence of 5 potential N-glycosylation sites. LPH alpha, which is rich in cysteine and hydrophobic amino acid residues, may fold rapidly into a tight and rigid globular domain in which carbohydrate attachment sites are no longer accessible to glycosyltransferases. When expressed independently in COS-1 cells, the LPH beta polypeptide forms a misfolded, transport-incompetent molecule. We propose a role for the LPH alpha domain within the pro-LPH molecule as an intramolecular chaperone during folding in the ER.