Malaria parasite cGMP-dependent protein kinase regulates blood stage merozoite secretory organelle discharge and egress.

The malaria parasite replicates within an intraerythrocytic parasitophorous vacuole (PV). Eventually, in a tightly regulated process called egress, proteins of the PV and intracellular merozoite surface are modified by an essential parasite serine protease called PfSUB1, whilst the enclosing PV and erythrocyte membranes rupture, releasing merozoites to invade fresh erythrocytes. Inhibition of the Plasmodium falciparum cGMP-dependent protein kinase (PfPKG) prevents egress, but the underlying mechanism is unknown. Here we show that PfPKG activity is required for PfSUB1 discharge into the PV, as well as for release of distinct merozoite organelles called micronemes. Stimulation of PfPKG by inhibiting parasite phosphodiesterase activity induces premature PfSUB1 discharge and egress of developmentally immature, non-invasive parasites. Our findings identify the signalling pathway that regulates PfSUB1 function and egress, and raise the possibility of targeting PfPKG or parasite phosphodiesterases in therapeutic approaches to dysregulate critical protease-mediated steps in the parasite life cycle

RhoD participates in the regulation of cell-cycle progression and centrosome duplication

We have previously identified a Rho protein, RhoD, which localizes to the plasma membrane and the early endocytic compartment. Here, we show that a GTPase-deficient mutant of RhoD, RhoDG26V, causes hyperplasia and perturbed differentiation of the epidermis, when targeted to the skin of transgenic mice. In vitro, gain-of-function and loss-of-function approaches revealed that RhoD is involved in the regulation of G1/S-phase progression and causes overduplication of centrosomes. Centriole overduplication assays in aphidicolin-arrested p53-deficient U2OS cells, in which the cell and the centrosome cycles are uncoupled, revealed that the effects of RhoD and its mutants on centrosome duplication and cell cycle are independent. Enhancement of G1/S-phase progression was mediated via Diaph1, a novel effector of RhoD, which we have identified using a two-hybrid screen. These results indicate that RhoD participates in the regulation of cell-cycle progression and centrosome duplication.Oncogene advance online publication, 4 June 2012; doi:10.1038/onc.2012.195

Synergistic role of fission yeast Alp16 GCP6

In fission yeast, γ-tubulin ring complex (γTuRC)–specific components Gfh1(GCP4), Mod21(GCP5), and Alp16(GCP6) are nonessential for cell growth. Of these deletion mutants, only alp16Δ shows synthetic lethality with temperature-sensitive mutants of Mzt1(MOZART1), a component of the γTuRC required for recruitment of the complex to microtubule-organizing centers. γ-Tubulin small complex levels at mitotic spindle pole bodies (SPBs, the centrosome equivalent in fungi) and microtubule levels for preanaphase spindles are significantly reduced in alp16Δ cells but not in gfh1Δ or mod21Δ cells. Furthermore, alp16Δ cells often form monopolar spindles and frequently lose a minichromosome when the spindle assembly checkpoint is inactivated. Alp16(GCP6) promotes Mzt1-dependent γTuRC recruitment to mitotic SPBs and enhances spindle microtubule assembly in a manner dependent on its expression levels. Gfh1(GCP4) and Mod21(GCP5) are not required for Alp16(GCP6)-dependent γTuRC recruitment. Mzt1 has an additional role in the activation of the γTuRC for spindle microtubule assembly. The ratio of Mzt1 to γTuRC levels for preanaphase spindles is higher than at other stages of the cell cycle. Mzt1 overproduction enhances spindle microtubule assembly without affecting γTuRC levels at mitotic SPBs. We propose that Alp16(GCP6) and Mzt1 act synergistically for efficient bipolar spindle assembly to ensure faithful chromosome segregation