Proline 36 of the Factor XIII Activation Peptide Plays a Crucial Role in Substrate Recognition and Zymogen Activation

Factor XIII Activation Peptide Residues Play Important Roles in Stability, Activation, and Transglutaminase Activity

A subunit of factor XIII (FXIII-A) contains a unique activation peptide (AP) that protects the catalytic triad and prevents degradation. In plasma, FXIII is activated proteolytically (FXIII-A*) by thrombin and Ca2+ cleaving AP, while in cytoplasm, it is activated nonproteolytically (FXIII-A°) with increased Ca2+ concentrations. This study aimed to elucidate the role of individual parts of the FXIII-A AP in protein stability, thrombin activation, and transglutaminase activity. Recombinant FXIII-A AP variants were expressed, and SDS-PAGE was used to monitor thrombin hydrolysis at the AP cleavage sites R37-G38. Transglutaminase activities were assessed by cross-linking lysine mimics to Fbg αC (233-425, glutamine-substrate) and monitoring reactions by mass spectrometry and in-gel fluorescence assays. FXIII-A AP variants, S19P, E23K, and D24V, degraded during purification, indicating their vital role in FXIII-A2 stability. Mutation of P36 to L36/F36 abolished the proteolytic cleavage of AP and thus prevented activation. FXIII-A N20S and P27L exhibited slower thrombin activation, likely due to the loss of key interdomain H-bonding interactions. Except N20S and P15L/P16L, all activatable FXIII-A* variants (P15L, P16L, S19A, and P27L) showed similar cross-linking activity to WT. By contrast, FXIII-A° P15L, P16L, and P15L/P16L had significantly lower cross-linking activity than FXIII-A° WT, suggesting that loss of these prolines had a greater structural impact. In conclusion, FXIII-A AP residues that play crucial roles in FXIII-A stability, activation, and activity were identified. The interactions between these AP amino acid residues and other domains control the stability and activity of FXIII.

Novel Insights into Heterozygous Factor XIII Deficiency

The prevalence and clinical significance of heterozygous factor XIII (FXIII) deficiency has long been debated, with controversial reports emerging since 1988. In the absence of large epidemiologic studies, but based on a few studies, a prevalence of 1 per 1,000 to 5,000 is estimated. In southeastern Iran, a hotspot area for the disorder, a study of more than 3,500 individuals found an incidence of 3.5%. Between 1988 and 2023, a total of 308 individuals were found with heterozygous FXIII deficiency, of which molecular, laboratory, and clinical presentations were available for 207 individuals. A total of 49 variants were found in the F13A gene, most of which were missense (61.2%), followed by nonsense (12.2%) and small deletions (12.2%), most occurring in the catalytic domain (52.1%) of the FXIII-A protein and most frequently in exon 4 (17%) of the F13A gene. This pattern is relatively similar to homozygous (severe) FXIII deficiency. In general, heterozygous FXIII deficiency is an asymptomatic condition without spontaneous bleeding tendency, but it can lead to hemorrhagic complications in hemostatic challenges such as trauma, surgery, childbirth, and pregnancy. Postoperative bleeding, postpartum hemorrhage, and miscarriage are the most common clinical manifestations, while impaired wound healing has been rarely reported. Although some of these clinical manifestations can also be observed in the general population, they are more common in heterozygous FXIII deficiency. While studies of heterozygous FXIII deficiency conducted over the past 35 years have shed light on some of the ambiguities of this condition, further studies on a large number of heterozygotes are needed to answer the major questions related to heterozygous FXIII deficiency.

Transglutaminase Activities of Blood Coagulant Factor XIII Are Dependent on the Activation Pathways and on the Substrates

Factor XIII (FXIII) catalyzes formation of γ-glutamyl-ε-lysyl crosslinks between reactive glutamines (Q) and lysines (K). In plasma, FXIII is activated proteolytically (FXIII-A*) by the concerted action of thrombin and Ca2+. Cellular FXIII is activated nonproteolytically (FXIII-A°) by elevation of physiological Ca2+ concentrations. FXIII-A targets plasmatic and cellular substrates, but questions remain on correlating FXIII activation, resultant conformational changes, and crosslinking function to different physiological substrates. To address these issues, the characteristics of FXIII-A* versus FXIII-A° that contribute to transglutaminase activity and substrate specificities were investigated. Crosslinking of lysine mimics into a series of Q-containing substrates were measured using in-gel fluorescence, mass spectrometry, and UV-Vis spectroscopy. Covalent incorporation of fluorescent monodansylcadaverine revealed that FXIII-A* exhibits greater activity than FXIII-A° toward Q residues within Fbg αC (233-425 WT, Q328P Seoul II, and Q328PQ366N) and actin. FXIII-A* and FXIII-A° displayed similar activities toward α2-antiplasmin (α2AP), fibronectin, and Fbg αC (233-388, missing FXIII-binding site αC 389-402). Furthermore, the N-terminal α2AP peptide (1-15) exhibited similar kinetic properties for FXIII-A* and FXIII-A°. MALDI-TOF mass spectrometry assays with glycine ethyl ester and Fbg αC (233-425 WT, αC E396A, and truncated αC (233-388) further documented that FXIII-A* exerts greater benefit from the αC 389-402 binding site than FXIII-A°. Conformational properties of FXIII-A* versus A° are proposed to help promote transglutaminase function toward different substrates. A combination of protein substrate disorder and secondary FXIII-binding site exposure are utilized to control activity and specificity. From these studies, greater understandings of how FXIII-A targets different substrates are achieved.

Genetic landscape in coagulation factor XIII associated defects – Advances in coagulation and beyond

Coagulation factor XIII (FXIII) acts as a fine fulcrum in blood plasma that maintains the balance between bleeding and thrombosis by covalently crosslinking the pre-formed fibrin clot into an insoluble one that is resistant to premature fibrinolysis. In plasma, FXIII circulates as a pro-transglutaminase complex composed of the dimeric catalytic FXIII-A encoded by the F13A1 gene and dimeric carrier/regulatory FXIII-B subunits encoded by the F13B gene. Growing evidence accumulated over decades of exhaustive research shows that not only does FXIII play major roles in both pathological extremes of hemostasis i.e. bleeding and thrombosis, but that it is, in fact, a pleiotropic protein with physiological roles beyond coagulation. However, the current FXIII genetic-epidemiological literature is overwhelmingly derived from the bleeding pathology associated with its deficiency. In this article we review the current clinical, functional, and molecular understanding of this fascinating multifaceted protein, especially putting into the same perspective its genetic landscape.

Identification of amino acid residues that are crucial for FXIII-A intersubunit interactions and stability

Coagulation factor XIII (FXIII) is the main stabilizer of the fibrin clot. It circulates in plasma as a tetramer of two A-subunits and two B-subunits. Under physiological conditions, FXIII-A exists as a dimer (FXIII-A2). The interactions between the FXIII-A-subunits that stabilize the FXIII-A2 dimer are not fully understood. We therefore designed a systematic approach to identify amino acid residues crucial for the expression and stability of FXIII-A2. Based on the available FXIII-A2 crystal structure, we identified 12 amino acid residues forming intersubunit salt bridges and 21 amino acid residues forming hydrogen bonds between the two A-subunits. We chose 10 amino acid residues that form 5 particularly strong interactions, performed site-directed mutagenesis, and expressed the mutants in CHO cells. Disruption of these interactions by single mutation of Lys257, Lys113, Asp343, Glu401, or Asp404 abolished the expression of properly folded, soluble, and functional FXIII-A in CHO cells. On the contrary, mutation of Glu111, Arg100, or Asn112 had no significant effect on FXIII-A expression. Our results suggest that 4 intersubunit interactions (Arg11-Asp343, Lys113-Asp367, Lys257-Glu401, and Arg260-Asp404) are essential for the stability of FXIII-A2. Our findings are supported by reported mutations at Lys257, Arg260, and Asp404 found in patients with congenital FXIII-A deficiency.