A 110-amino acid region within the A1-domain of coagulation factor VIII inhibits secretion from mammalian cells.

Factor VIII is the coagulation factor deficient in the X-chromosome-linked bleeding disorder hemophilia A. Factor VIII is homologous to blood coagulation factor V, both having a domain structure of A1-A2-B-A3-C1-C2. Previous transfection studies demonstrated that factor VIII is 10-fold less efficiently expressed than the homologous coagulation factor, factor V. The inefficient expression correlated with interaction of the factor VIII primary translation product with the protein chaperonin BiP in the lumen of the endoplasmic reticulum. In contrast, factor V was not detected in association with BiP and was secreted efficiently. To determine whether specific amino acid sequences within factor VIII inhibit secretion, we have studied the secretion of factor VIII deletion and factor VIII/factor V chimeric proteins upon transient transfection of COS-1 monkey cells. A chimeric factor VIII protein that contained the A1- and A2-domains of factor V was secreted with a similar efficiency as wild-type factor V, whereas the complementary chimera having the A1- and A2-domains of factor VIII was secreted with low efficiency, similar to wild-type factor VIII. These results suggested that sequences within the A1- and A2-domains were responsible for the low secretion efficiency of factor VIII. Secretion of A1-domain-deleted factor VIII was increased approximately 10-fold compared to wild-type factor VIII or A2-domain-deleted factor VIII. Expression of the factor VIII A1-domain alone did not yield secreted protein, whereas expression of the factor VIII A2-domain alone or the factor V A1-domain or A2-domain alone directed synthesis of secreted protein. Secretion of a hybrid in which the carboxyl-terminal 110 amino acids of the A1-domain were replaced by homologous sequences from the factor V A1-domain was also increased 10-fold compared to wild-type factor VIII, however, the secreted protein was not functional and the heavy and light chains were not associated. These results localize a 110-amino acid region within the A1-domain that inhibits factor VIII secretion. This region is clustered with multiple short peptide sequences that have potential to bind BiP.