CENTRIN IS A CONSERVED PROTEIN THAT FORMS DIVERSE ASSOCIATIONS WITH CENTRIOLES AND MTOCS IN NAEGLERIA AND OTHER ORGANISMS

Centrin, a approximate to 20 kDa calcium-binding protein also known as caltractin, is a component of centrosome-associated algal flagellar r oots capable of calcium-mediated contraction, and is also found in the centrosomes of vertebrate cells. Our analysis of a centrin gene from a protist, the amoeboflagellate Naegleria gruberi, reveals conserved f eatures that distinguish centrins from calmodulin. Antibodies to bacte rially expressed Naegleria centrin, which also recognize yeast Cdc31p, were employed to localize centrin immunoreactivity in selected organi sms possessing specialized microtubule-organizing centers (MTOCs) or a ccessory structures. There is a striking morphological diversity of su ch structures. In the simplest associations, as found in Naegleria fla gellates and vertebrate tracheal epithelium, centrin is intimately ass ociated with the cylinder of the basal bodies. In cells with unfocused mitotic spindles, Naegleria amoebae and onion root tips, no localizat ion of centrin was detected. In Dictyostelium discoideum and Saccharom yces cerevisiae, which lack centrioles, centrin immunoreactivity was o bserved as punctate cytoplasmic bodies but not associated with spindle pole MTOCs. In Paramecium multimicronucleatum, centrin immunoreactivi ty is localized to the infraciliary lattice, previously shown to exhib it calcium-mediated contraction. In Vorticella microstoma, known for t he calcium-induced rapid contraction of its stalk, centrin immunoreact ivity is localized to the contractile spasmoneme and myonemes. Similar antigens from Paramecium and Vorticella are detected by anti-centrin and anti-spasmin. The pattern of localization of centrin immunoreactiv ity supports the conjecture that a contractile system involving centri n, initially associated with centriolar structures, was recruited duri ng evolution to build specialized organelles in different organisms an d cell types. (C) 1996 Wiley-Liss, Inc.