Non-proteolytic activation of cellular protransglutaminase (placenta macrophage factor XIII)

Abstract

Plasma Factor XIII is a zymogen (plasma protransglutaminase) with the tetrametric structure A2B2, whereas the cellular protransglutaminase, i.e. Factor XIII in the platelet and monocyte/macrophage, consists exclusively of A subunits (A2). It is generally accepted that at Ca2+ concentrations comparable with that in plasma the proteolytic removal of an Nterminal activation peptide is the prerequisite for the Ca2+-induced formation of a catalytically active configuration of subunit A. In this study it was demonstrated that at high concentrations NaCl or KCI induced a non-proteolytic activation of cellular (placental macrophage) but not plasma protransglutaminase. The activation depended on time and salt concentration, and Ca2 , in the range 0-20 mm, greatly enhanced the activation process. At 1.25 M-NaCl maximal activation occurred within 60 min in the presence of 2 mM-CaC12, and even at physiological NaCl concentration a slow progressive activation could be observed in the presence of Ca2+. The specific activity of salt-activated Factor XIII was 1.5-2.0-fold higher than that obtained after thrombin activation. The non-proteolytic activation of cellular protransglutaminase was abolished by the addition of subunit B of plasma Factor XIII in stoichiometric amount, which suggests that (one of) the physiological function(s) of the B subunit in plasma Factor XIII is to prevent the slow spontaneous activation of A subunit that would occur in a plasmatic environment. INTRODUCTIONFactor XIII (FXIII) of blood coagulation present in the plasma is a zymogen (plasma protransglutaminase) of tetrameric structure (A2B2). The active site formed in the course of an activation process is located on the A subunit while subunit B remains enzymically inactive. The presence of FXIII has also been verified in platelets [1], monocytes and monocyte-derived macrophages [2][3][4][5][6][7]. In contrast with the plasma FXIII, however, this cellular protransglutaminase consists exclusively of A subunits (A2). The active transglutaminase (FXIIIa) formed from FXIII catalyses an acyl-transfer reaction in which the carboxamide group of a peptide-bound glutamine residue is the acyl

Possible role for cellular FXIII in monocyte-derived dendritic cell motility

“…Consistent with these data, the migratory response of FXIII-A-deficient DCs was reduced compared to that of control cells. It is known that intracellular rise of Ca 2+ concentration is sufficient to trigger activation of FXIII (Polgar et al, 1990). Even though a rise in intracellular calcium appears to be dispensable for DC directional migration (Li et al, 2000), the strong and sustained calcium mobilization induced by CCL19 (Humrich et al, 2006) may be directly linked to the role of FXIII-A in DC motility.…”

“…The A subunit dimer, FXIII-A 2 , functions as an intracellular TG in the megakaryocyte/ platelet and monocyte/macrophage lineages where it can be activated by low Ca 2+ concentrations in the absence of thrombin (Polgar et al, 1990;Siebenlist et al, 2001;Adany and Bardos, 2003). Platelets contain large amounts of the FXIII-A while the concentration in monocytes is at least one magnitude less than that in platelets (Muszbek et al, 1999).…”

Reversible Activation of Cellular Factor XIII by Calcium

“…The Ca 2ϩ dependence of the FXIIIa°production can be modulated by several factors because addition of chaotropic ions such as KSCN, p-toluenesulfonate, iodide, or bromide all lowered the Ca 2ϩ concentration required for activation (18). Only 2 mM CaCl 2 was required to induce FXIIIa°in the presence of 1 M NaCl or KCl (14). This is still higher than the net intracellular Ca 2ϩ concentration of 10 Ϫ4 mM in e.g.…”

“…Being located intracellularly, cFXIII is not accessible to cleavage by thrombin. Several reports have demonstrated that cFXIII is activated without cleavage of the activation peptide, resulting in the FXIIIa°molecule (12)(13)(14). Furthermore, this nonproteolytic activation of FXIII has been demonstrated to occur in cells e.g.…”

“…Higher concentrations are, however, reached during Ca 2ϩ fluxes induced as a result of various stimuli (19,21,22), and FXIIIa°is indeed generated in the cytosols of monocytes or activated platelets (12,13,23). FXIII-B subunits appear to prevent the nonproteolytic activation of pFXIII in plasma as indicated by in vitro experiments showing that titration of cFXIII with stoichiometric amounts of B subunit strongly inhibits the activation induced by NaCl/Ca 2ϩ (14). It is obvious that FXIIIa* generated as a result of proteolytic activation cannot revert to zymogen FXIII.…”