Decoding full-length factor VIII through the structural and functional lens of its B domain.

The factor VIII (FVIII) B domain, a large and heavily glycosylated region, is crucial for FVIII secretion, although its structural and functional roles remain incompletely understood. Although the B domain is dispensable for cofactor activity, previous research hints at multiple, yet unverified, functional roles. Here, we used an integrative hybrid approach to generate detailed structural models of glycosylated and furin-cleaved full-length FVIII (Gly/FC-FL-FVIII) that include the B domain, using AlphaFold2 predictions. These models were further refined and validated with biophysical data from atomic force microscopy (AFM) and cryogenic electron microscopy of recombinant and plasma-derived FL-FVIII proteins. Additionally, we modeled a FL-FVIII-von Willebrand factor (VWF) complex with both the B domain and D'-D3 domains to investigate B-domain interactions during intracellular processing. Our results suggest that the B domain functions largely as an independent domain encircling the FVIII core. AFM studies revealed that VWF tails are associated with globular FVIII structures. Structural analysis indicated that B-domain glycosylation enhances stability, supports proper folding, and promotes efficient trafficking through the endoplasmic reticulum-Golgi pathway. We propose that, within the Golgi, the B domain may unfold around FVIII via furin cleavages, facilitating essential interactions with partners such as VWF. Our models also suggest that the B domain shields key FVIII epitopes, potentially reducing immune interference and inhibiting premature activation. This study presents a novel structural model of the FVIII B domain and FL-FVIII, offering critical insights into previously unexplored aspects of B-domain function and its essential role in FVIII biology.