Molecular and cellular basis of deficiency of the b subunit for factor XIII secondary to a Cys430-Phe mutation in the seventh Sushi domain.

We studied the defect responsible for deficiency of the b subunit for factor XIII in the first known case of this condition. The patient is a compound heterozygote of two genetic defects: deletion of A-4161 at the acceptor splice junction of intron A, resulting in a loss of the obligatory AG splicing sequence; and, replacement of G-11499 by T in exon VIII, resulting in an amino acid substitution of Cys430 by Phe. To determine how the latter mutation impaired b subunit synthesis, recombinant b subunit bearing the mutation was expressed in BHK cells. The mutant as well as wild-type b subunit was synthesized by the cells. However, the apparent molecular weight of the mutant was slightly higher than those of the wild-type and plasma b subunits under nonreducing conditions, probably because of destruction of a disulfide bond. The mutant b subunit was secreted from the cells much less effectively than the wild type and remained susceptible to endoglycosidase H, indicating that it was not transported from the endoplasmic reticulum to the Golgi apparatus where the processing of oligosaccharides occurs. Immunofluorescence study suggested that the mutant protein was retained in the endoplasmic reticulum. These studies demonstrate that a Cys430-Phe mutation does not prevent the de novo synthesis of the b subunit, but alters the conformation of the mutant protein sufficiently to impair its intracellular transport, resulting in its deficiency in this patient.