Functional roles of the factor VIII B domain
Unravelling the structure, function and molecular interactions of factor VIII (FVIII) throughout its life cycle from biosynthesis to clearance has advanced our understanding of the molecular mechanisms of haemophilia and the development of effective treatment strategies including recombinant replacement therapy. These insights are now influencing bioengineering strategies toward novel therapeutics. Whereas available molecular models and crystal structures have helped elucidate the structure and function of the A and C domains of FVIII, these models have not included detailed structural information of the B domain. Therefore, insights into the role of the FVIII B domain have come primarily from expression studies in heterologous systems, biochemical studies on bioengineered FVIII variants and clinical studies with B domain-deleted FVIII. This manuscript reviews the available data on the potential functional roles of the FVIII B domain. A detailed literature search was performed, and the data extracted were qualitatively summarized. Intriguing emerging evidence suggests that the FVIII B domain is involved in intracellular interactions that regulate quality control and secretion, as well as potential regulatory roles within plasma during activation, platelet binding, inactivation and clearance.