Nucleus-vacuole junctions and piecemeal microautophagy of the nucleus in S. cerevisiae

Various modes of autophagy conspire to degrade virtually every compartment of the eukaryotic cell. In Saccharomyces cerevisiae, a process called piecemeal microautophagy of the nucleus (PMN) even pinches off and degrades nonessential portions of the nucleus. PMN is a constitutive process induced to high levels by starvation or rapamycin, an inhibitor of TOR kinase. PMN occurs at nucleus-vacuole (NV) junctions, which are Velcro-like patches formed by interactions between the vacuole membrane protein Vac8p and the outer-nuclear-membrane protein Nvj1p. In response to nutrient depletion, Nvj1p increasingly binds and sequesters two proteins with roles in lipid metabolism, Osh1p and Tsc13p. Tsc13p is required for the normal biogenesis of PMN vesicles. The sequestration of Osh1p by Nvj1p likely serves to negatively regulate the trafficking of tryptophan permease(s) to the plasma membrane. Thus, NV junctions and PMN orchestrate novel and sophisticated responses to nutrient limitation.