INEFFICIENT MEMBRANE TARGETING, TRANSLOCATION, AND PROTEOLYTIC PROCESSING BY SIGNAL PEPTIDASE OF A MUTANT PREPROPARATHYROID HORMONE PROTEIN

A preproparathyroid hormone allele from a patient with familial isolat ed hypoparathyroidism was shown to have a single point mutation in the hydrophobic core of the signal sequence, This mutation, changing a cy steine to an arginine codon at the -8 position of the signal peptide, was associated with deleterious effects on the processing of prepropar athyroid hormone to proparathyroid hormone in vitro, To examine the bi ochemical consequence(s) of this mutation, proteins produced by cell-f ree translation of wild type and mutant cRNAs were used in assays that reconstitute the early steps of the secretory pathway, We find that t he mutation impairs interaction of the nascent protein with signal rec ognition particle and the translocation machinery, Moreover, cleavage of the mutant signal sequence by solubilized signal peptidase is ineff ective, The consequence of this mutation on processing and secretion o f parathyroid hormone is confirmed in intact cells by pulse-chase expe riments following transient expression of the mutant protein in COS-7 cells. The inability of the mutant signal sequence, however, to interf ere with the targeting and processing of other secreted proteins does not support obstruction of the translocation apparatus as the mechanis m underlying the dominant mode of inheritance of hypoparathyroidism in this family.