Over the last two decades, it became evident that factor XIII (FXIII) is not only a crucial determinant of clot characteristics but also has potentially important functions in many various fields such as bone biology, immunity, and adipogenesis. In this review, we aim to summarize the latest findings regarding structure and function of FXIII. In regard to FXIII structure, much progress has been made recently to understand how its subunits are held together. In the A subunit, the activation peptide has a crucial role in the formation of FXIII-A2 dimers. In the B subunit, Sushi domains that are involved in binding to the A subunit and in B2 dimer formation have been identified. In regard to FXIII function, interactions with immune cells and the complement system have been described. A novel function of FXIII-A in adipogenesis has been suggested. The role of FXIII-A in osteoblast differentiation has been further investigated; however, a novel double knockout mouse deficient in both FXIII-A and transglutaminase 2 showed normal bone formation. Thus, more research, in particular, into the cellular functions of FXIII-A is still required.
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