Multiple functions of the vacuolar sorting protein Ccz1p in Saccharomyces cerevisiae

The Ccz1 mediates the autophagic clearance of damaged mitochondria in response to oxidative stress in Candida albicans

Autophagy plays a critical role in response to numerous cellular stresses, such as nutrient deprivation, hypoxia, starvation and organelle damage. The disruption of autophagy pathway affects multiple aspects of cellular stress response. Here we for the first time identified Ccz1 as an essential component for autophagy in Candida albicans. Our experiments demonstrated that loss of CCZ1 gene led to vacuolar fragmentation and disruption of the autophagy pathway. Our results also suggested that Ccz1 functioned in oxidative stress. In the ccz1Δ/Δ mutant, the levels of reactive oxidative species (ROS) sharply increased under H2O2 treatment. Further studies demonstrated that breakdown of the autophagic clearance pathway led to the accumulation of oxidative stress-damaged mitochondria, and consequently elevated cellular ROS levels in the ccz1Δ/Δ mutant. Furthermore, deletion of CCZ1 led to a significant defect in filamentous development at both 30°C and 37°C. The disruption of CCZ1 gene led to decreased capacity of macrophage killing and increased sensitivity to the macrophages. In addition, the ccz1Δ/Δ mutant exhibited attenuated virulence and decreased fungal burdens in the mouse systemic infection model, indicating that CCZ1 might provide a promising target for antifungal drugs development. In summary, our findings provide new insights into the understanding of autophagy-related gene in C. albicans.

Mon1a protein acts in trafficking through the secretory apparatus

Mon1a was originally identified as a modifier gene of vesicular traffic, as a mutant Mon1a allele resulted in increased localization of cell surface proteins, whereas reduced levels of Mon1a showed decreased secretory activity. Here we show that Mon1a affects different steps in the secretory pathway including endoplasmic reticulum-to-Golgi traffic. siRNA-dependent reduction of Mon1a levels resulted in a delay in the reformation of the Golgi apparatus after Brefeldin A treatment. Endoglycosidase H treatment of ts045VSVG-GFP confirmed that knockdown of Mon1a delayed endoplasmic reticulum-to-Golgi trafficking. Reductions in Mon1a also resulted in delayed trafficking from Golgi to the plasma membrane. Immunoprecipitation and mass spectrometry analysis showed that Mon1a associates with dynein intermediate chain. Reductions in Mon1a or dynein altered steady state Golgi morphology. Reductions in Mon1a delayed formation of ERGIC-53-positive vesicles, whereas reductions in dynein did not affect vesicle formation. These data provide strong evidence for a role for Mon1a in anterograde trafficking through the secretory apparatus.

On the fate of early endosomes

Proteins are endocytosed by various pathways into the cell. All these pathways converge at the level of the early endosome. The fate of the early endosome and how proteins are sorted into recycling and late endosomes/multi-vesicular body is a matter of debate and intense research. Obviously, the transition from early to late endosome poses an interesting logistic problem and would merit attention on an intellectual level. Numerous diseases are also caused by defects in turning off/over signaling molecules or mis-sorting of proteins at the level of the early endosome. This brief review aims to discuss different molecular mechanisms whereby early-to-late endosome transition could be achieved.

Yeast protein phosphatases Ptp2p and Msg5p are involved in G1-S transition, CLN2 transcription, and vacuole morphogenesis

We previously reported that double disruption of protein phosphatase (PPase) genes PTP2 (phosphotyrosine-specific PPase) and MSG5 (phosphotyrosine and phosphothreonine/serine-PPase) causes Ca(2+) sensitive growth, whereas the single disruptions do not. This finding suggests that Ptp2p and Msg5p are involved in Ca(2+)-induced stress response in a redundant manner. To gain insight into the molecular mechanism causing calcium sensitivity of the ptp2 msg5 double disruptant, we performed fluorescence-activated cell sorting analysis and found a delayed G1 phase. This delayed G1 was consistent with the defect in bud emergence, and reduced CLN2 transcription upon addition of CaCl(2). We also found that Slt2p is hyper-phosphorylated in the Deltaptp2 Deltamsg5 double disruptant and that the vacuole of the Deltaptp2 Deltamsg5 double disruptant is fragmented even in the absence of Ca(2+). These findings suggest that both Ptp2p and Msg5p are involved in the G1 to S transition and vacuole morphogenesis possibly through their regulation of Slt2 pathway.

Genetic variation in Mon1a affects protein trafficking and modifies macrophage iron loading in mice

We undertook a quantitative trait locus (QTL) analysis in mice to identify modifier genes that might influence the severity of human iron disorders. We identified a strong QTL on mouse chromosome 9 that differentially affected macrophage iron burden in C57BL/10J and SWR/J mice. A C57BL/10J missense allele of an evolutionarily conserved gene, Mon1a, cosegregated with the QTL in congenic mouse lines. We present evidence that Mon1a is involved in trafficking of ferroportin, the major mammalian iron exporter, to the surface of iron-recycling macrophages. Differences in amounts of surface ferroportin correlate with differences in cellular iron content. Mon1a is also important for trafficking of cell-surface and secreted molecules unrelated to iron metabolism, suggesting that it has a fundamental role in the mammalian secretory apparatus.

Pmr1, a Golgi Ca2+/Mn2+-ATPase, is a regulator of the target of rapamycin (TOR) signaling pathway in yeast

The rapamycin.FKBP12 complex inhibits target of rapamycin (TOR) kinase in TORC1. We screened the yeast nonessential gene deletion collection to identify mutants that conferred rapamycin resistance, and we identified PMR1, encoding the Golgi Ca2+/Mn2+ -ATPase. Deleting PMR1 in two genetic backgrounds confers rapamycin resistance. Epistasis analyses show that Pmr1 functions upstream from Npr1 and Gln-3 in opposition to Lst8, a regulator of TOR. Npr1 kinase is largely cytoplasmic, and a portion localizes to the Golgi where amino acid permeases are modified and sorted. Nuclear translocation of Gln-3 and Gln-3 reporter activity in pmr1 cells are impaired, but expression of functional Gap1 in the plasma membrane of a pmr1 strain in response to nitrogen limitation is enhanced. These two phenotypes suggest up-regulation of Npr1 function in the absence of Pmr1. Together, our results establish that Pmr1-dependent Ca2+ and/or Mn2+ ion homeostasis is necessary for TOR signaling.