The biological function of the carboxy-terminal portion of the PTH molecule is unknown. We hypothesized that the carboxy-terminus of PTH may be essential for hormone processing and secretion. To ascertain the potential role of the carboxy-terminus in transport through the secretory pathway, we constructed a series of carboxy- terminally deleted mutants of human preproPTH. PreproPTH and these truncated peptides were expressed in cell-free extracts and in acutely transfected COS 7 cells. Intact preproPTH(l-84) (115 residues) was processed to proPTH(l-84); the PTH was secreted very efficiently. In contrast, translocation across the membrane of the endoplasmic reticulum and signal sequence cleavage were mildly impaired for PTH(l-52) (83 residues), moderately impaired for PTH(l-40) (71 residues), and dramatically impaired for PTH(l-34) (65 residues). Subsequent cleavage of the prosequence and secretion from intact cells were also dramatically impaired for PTH(l-52) and undetectable for PTH(l-40) and PTH(l-34). We conclude that more than 65 residues are needed to cross the endoplasmic reticulum and that more than 83 residues are needed for efficient prosequence cleavage and secretion. Perhaps similar requirements for full transport through the secretory pathway explains why most secreted peptides are synthesized as long propeptides of at least 80 residues.
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