ROLE OF THE B-DOMAIN FOR FACTOR-VIII AND FACTOR-V EXPRESSION AND FUNCTION

Factor V and factor VIII are homologous cofactors in the blood coagula tion cascade that have the domain structure A1-A2-B-A3-C1-C2, of which the B domain has extensively diverged, In transfected COS-1 monkey ce lls, expression of factor VIII is approximately 10-fold less efficient than that of factor V, primarily because of inefficient protein secre tion and, to a lesser extent, reduced mRNA expression. To study the fu nctional significance and effect of the B domain on expression and act ivity, chimeric cDNAs were constructed in which the B domains of facto r V and factor VIII were exchanged. Expression of a factor VIII chimer a harboring the B-domain of factor V yielded a fully functional factor VIII molecule that was expressed twofold more efficiently than wild-t ype factor VIII because of increased mRNA expression. Thus, sequences within the factor VIII B domain were not responsible for the inefficie nt secretion of factor VIII compared with factor V, Expression of a fa ctor V chimera harboring the B domain of factor VIII was slightly redu ced compared with wild-type factor V, although the secreted molecule h ad significantly reduced procoagulant activity correlating with dissoc iated heavy and light chains and resistance to thrombin activation, In terestingly, the factor V chimera containing the factor VIII B domain was efficiently activated by Russell's viper venum (RVV), A factor V B domain deletion (residues 710-1545) molecule also exhibited significa ntly reduced procoagulant activity caused by resistance to thrombin cl eavage and activation, although this molecule was activatable by RW. T hese results show that, in contrast to factor VIII, thrombin activatio n of factor V requires sequences within the B domain. In addition, thr ombin activation of factor V occurs through a different mechanism than activation by RVV. (C) 1994 by The American Society of Hematology,