Synthesis, intracellular distribution, and secretion of multiple forms of parathyroid secretory protein-I

Examination of whole cell extracts and subcellular fractions of dispersed porcine parathyroid cells incubated with [35S]methionine indicates that two species of secretory protein-I, 72,000 and 64,000 daltons, respectively, are synthesized. Two secretory protein-I species of molecular weights equivalent to those in the cell but with slightly different isoelectric points were secreted; calcium suppressed the secretion of both of these. The secretory protein-I of cell and medium were shown to be related to each other and to previously identified secreted secretory protein-I by comparison of their 35S-labeled tryptic peptides and location of methionine in positions 7, 15, and 32 of the peptide chains. Both of the cellular species appeared to be enclosed within membranes similar to those containing parathyroid hormone and its immediate biosynthetic precursor because they were associated with the membrane fraction of the cell, were not digested when the membranes were exposed to trypsin, and were extracted from these membranes, as were parathyroid hormone and proparathyroid hormone, with dilute sodium deoxycholate. We did not find an amino-terminal precursor form of secretory protein-I in an incubation as short as 2 min with [35S]methionine, whereas [35S]proparathyroid hormone was readily detected, indicating that processing of secretory protein-I involves a direct conversion of the pre-protein to the secretory protein-I. Posttranslational glycosylation or deletion of carboxy-terminal region of the secretory protein-I species might account for the differences in molecular weights and isoelectric points of the cellular and secreted forms.