LAUSR

Factor XIII (FXIII) is a transglutaminase circulating in human plasma as a tetramer consisting of two zymogen A subunits and two pro-tective/inhibitory B subunits. 1 Interestingly, FXIII is also present as dimer of its A- subunit in the cytosol of human platelets at a con-centration that is 150- fold greater than in plasma. 2 The existence of this pool of cytosolic FXIII (cFXIII) is intriguing because platelet activation by single agonists does not elicit its secretion 3 and its intracellular functions are not clearly elucidated yet. 1,4 Some light was recently shed by Mitchell et al., 5 who showed for the first time that cFXIII is externalized and retained on the membrane of platelets when they are simultaneously activated by collagen and thrombin. In this issue of the Journal of Thrombosis and Haemostasis , Somodi et al. 6 set out to explore the mechanisms underlying cFXIII externalization on platelets and platelet- derived microparticles. First, they showed that a prolonged high cytosolic free calcium ([Ca 2 + ] cyt ) level is necessary but not sufficient for translocating cFXIII to the surface of