Building centrioles

he protozoan parasite Toxoplasma gondii heads out of its host cell when Ca 2 ϩ levels peak in its cytoplasm. Now, Arrizabalaga et al. (page 653) find that proper regulation of these Ca 2 ϩ levels, and thus of Toxoplasma exit, requires a Na ϩ /H ϩ exchanger (NHE), suggesting a link between proton and Ca 2 ϩ regulation. The increase in Ca 2 ϩ levels, which can be induced artificially with an ionophore, triggers a series of morphological changes that primes Toxoplasma for both exit and a speedy entrance into a neighboring cell. Death can ensue, however, if the ionophore sticks around while there is no target cell for the parasite to enter; presumably the over-stimulated Toxoplasma cannot sustain itself in an activated state. The authors used this death assay to isolate a mutant defective in activation. One mutant with a disruption in a sodium/ hydrogen exchanger (NHE) showed both delayed exit and reduced Toxoplasma cell death after sustained ionophore exposure. Toxoplasma lacking this plasma membrane pump also had normal internal pH, but their proton efflux was no longer prevented with an NHE inhibitor. Resting Ca 2 ϩ levels were higher in mutant parasites. This would make it harder to detect any exit signal that increased these Ca 2 ϩ levels even further, perhaps explaining why these mutants are slow to exit. The increase in resting Ca 2 ϩ levels may come about if a Ca 2 ϩ exporter (Ca 2 ϩ out/H ϩ in) relies on prior action of the NHE (H ϩ out/ Na ϩ in). Such a hypothetical linkage may be part of regulating Toxoplasma mobility or simply an intersection between the exit system and Toxoplasma 's normal ion homeostasis. ᭿ T Toxoplasma need a plasma membrane ion exchanger (right) for a speedy exit. Building centrioles entrioles and surrounding centrosomal material nucleate both mitotic spindles and interphase microtubule asters, but centri-oles are not an easy structure to dissect with either biochemistry or genetics. There are budding yeast mutants that arrest at various stages of maturation of the spindle pole body (SPB), but the SPB and mammalian centriole bear no physical resemblance to each other despite sharing some proteins and fulfilling the same function. Matsuura et al. (page 663) therefore resort to a trick of Chlamydomonas biology. In this unicellular green algae, the basal C Bld10p (gold particles) localizes to the proximal part of the basal body …