ULK1-mediated phosphorylation regulates the conserved role of YKT6 in autophagy

Autophagy is a catabolic process during which cytosolic material is enwrapped in a newly formed double-membrane structure called the autophagosome, and subsequently targeted for degradation in the lytic compartment of the cell. The fusion of autophagosomes with the lytic compartment is a tightly regulated step and involves membrane-bound SNARE proteins. These play a crucial role as they promote lipid mixing and fusion of the opposing membranes. Among the SNARE proteins implicated in autophagy, the essential SNARE protein YKT6 is the only SNARE protein that is evolutionarily conserved from yeast to humans. Here, we show that alterations in YKT6 function, in both mammalian cells and nematodes, produce early and late autophagy defects that result in reduced survival. Moreover, mammalian autophagosomal YKT6 is phospho-regulated by the ULK1 kinase, preventing premature bundling with the lysosomal SNARE proteins and thereby inhibiting autophagosome-lysosome fusion. Together, our findings reveal that timely regulation of the YKT6 phosphorylation status is crucial throughout autophagy progression and cell survival.

Atg1 kinase regulates autophagosome-vacuole fusion by controlling SNARE bundling

Autophagy mediates the degradation of cytoplasmic material. Upon autophagy induction, autophagosomes form a sealed membrane around the cargo and fuse with the lytic compartment to release the cargo for degradation. In order to avoid premature fusion of immature autophagosomal membranes with the lytic compartment, this process needs to be tightly regulated. Several factors mediating autophagosome–vacuole fusion have recently been identified. In budding yeast, autophagosome–vacuole fusion requires the R‐SNARE Ykt6 on the autophagosome, together with the three Q‐SNAREs Vam3, Vam7, and Vti1 on the vacuole. However, how these SNAREs are regulated during the fusion process is poorly understood. In this study, we investigate the regulation of Ykt6. We found that Ykt6 is directly phosphorylated by Atg1 kinase, which keeps this SNARE in an inactive state. Ykt6 phosphorylation prevents SNARE bundling by disrupting its interaction with the vacuolar SNAREs Vam3 and Vti1, thereby preventing premature autophagosome–vacuole fusion. These findings shed new light on the regulation of autophagosome–vacuole fusion and reveal a further step in autophagy controlled by the Atg1 kinase.

The Many Faces of Carbohydrate Metabolism in Male Germ Cells: From Single Molecules to Active Polymers

Spermatogenesis is a complex physiological process that involves cell proliferation, meiotic division and a final cell differentiation of post-meiotic cells into spermatozoa. During this process male germ cells also undergo a metabolic differentiation process, in which post-meiotic spermatogenic cells (spermatids) but not meiotic spermatogenic cells (spermatocytes) respond differentially to D-glucose metabolism, glucose transporters (GLUTs) distribution and utilization of non-hexose substrates, such as lactate/pyruvate or dihydroxyacetone. These differences might be explained by the requirement for a specific metabolic process to support cell differentiation or in some cases, cell viability. In addition, though glycogen is considered to be the main glucose store, in male germ cells this polymer may play a novel role in cell proliferation, acting as a new marker for apoptotic events in testicular tissue via a yet unknown mechanism. In this article, we summarize the main metabolic changes that occur during male germ differentiation, with a specific focus on metabolic sources during spermatocyte to spermatid transition. The latter considering that these cells come from the same cell linage as specialized cells, but are not isolated from their environment, describing the roles from single molecules to polymers on the viability of male germ cells.