Project Summary
Factor XIII is found in quite different locations including plasma (A2B2), the cellular (A2) of platelets,
megakaryocytes, and monocytes, and (A2) in bone extracellular matrix. Plasma FXIII A2B2 is activated
proteolytically by thrombin and low mM Ca2+ whereas FXIII A2 is activated nonproteolytically by a range of Ca2+
concentrations. The resultant transglutaminases (FXIII A* and A°) introduce covalent ¿-glutamyl-e-lysyl
crosslinks into protein networks. The main objective of our research is to decipher the kinetic and
conformational differences between the two activated forms of Factor XIII (FXIII) and to characterize their
ability to affect fibrin clot architecture and related processes. FXIII has roles in hemostasis, venous thrombosis,
cardiac repair, bone remodeling, innate immunity, etc. There are, however, few examples of strategies to
therapeutically target FXIII. To further enhance such developments, critical gaps in knowledge need to be
addressed. We need to know more about 1) the substrate specificities and structural features of activated
FXIII species from different activation pathways and 2) how fibrin clot architecture is controlled by FXIII
catalyzed crosslinking reactions involving the fibrin aC region and ¿ module. The proposed research will thus
address two hypotheses: 1) Factor XIII is hypothesized to have optimized activation pathways, protein
structural features, and substrate specificities to effectively target its actions in different physiological
environments. Research will focus a) on the kinetic properties of proteolytic FXIII A* and nonproteolytic FXIII
A°, b) on probing conformational roles of FXIII Activation Peptide interactions, and c) on enhancing calcium
binding. 2) The fibrin(ogen) aC region and ¿ module are hypothesized to enhance their interactions with FXIII
by utilizing distinct reactive glutamine environments and employing binding segments for FXIII. a) With aC
(233-425), we will focus on quantitatively exploring how individual reactive Qs will facilitate the reactivities of
remaining Qs. b) With ¿-module (142-411), we will rank the reactivities of Q398/Q399 and assess the role that
anchoring sites plays in controlling substrate specificity. The biochemical methods required to address these
aims include activity assays, fibrin crosslinking gels, mass spectrometry, solution NMR, and analytical
ultracentrifugation. Our contributions to this research field will have a positive impact because they will help to
address unresolved issues associated with FXIII structure-function, and as a result, lead to new innovative
strategies to manage FXIII in particular physiological locations