Human factor XSanto Domingo is a form of coagulation factor X in which a mutation within the signal peptide region of the precursor protein has been correlated genetically with a severe deficiency of factor X in the affected individual. A point mutation results in substitution of Arg for Gly at the critical -3 position of the factor X signal peptide. To determine the biochemical effect of this mutation on the biosynthesis of factor X, the wild-type and mutant factor X cDNAs were subcloned into a vector for transcription and translation in vitro. Translation products of mRNAs encoding portions of both mutant and wild-type proteins were used in a systematic biochemical approach to evaluate directly the effect of the mutation on targeting, transport, and proteolytic processing in vitro. The results show that targeting and transport of factor XSanto Domingo to the endoplasmic reticulum are functionally dissociated from the removal of the signal peptide by signal peptidase. Factor XSanto Domingo is translocated into the endoplasmic reticulum but not processed by signal peptidase. Transient expression of the wild-type and mutant factor X in human embryonic kidney 293 cells revealed apparently normal secretion of the glycosylated two-chain form of factor X but no secretion of factor XSanto Domingo. Thus, the inability of signal peptidase to cleave factor XSanto Domingo is directly responsible for the absence of circulating factor X and leads to the bleeding diathesis in the affected individual.
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