MIT domain of Vps4 is a Ca2+-dependent phosphoinositide-binding domain

The microtubule interacting and trafficking (MIT) domain is a small protein module that is conserved in proteins of diverged function, such as Vps4, spastin and sorting nexin 15 (SNX15). The molecular function of the MIT domain is protein-protein interaction, in which the domain recognizes peptides containing MIT-interacting motifs. Recently, we identified an evolutionarily related domain, 'variant' MIT domain at the N-terminal region of the microtubule severing enzyme katanin p60. We found that the domain was responsible for binding to microtubules and Ca(2+). Here, we have examined whether the authentic MIT domains also bind Ca(2+). We found that the loop between the first and second α-helices of the MIT domain binds a Ca(2+) ion. Furthermore, the MIT domains derived from Vps4b and SNX15a showed phosphoinositide-binding activities in a Ca(2+)-dependent manner. We propose that the MIT domain is a novel membrane-associating domain involved in endosomal trafficking.