Proteasomes are highly abundant protein complexes that are responsible for most regulated protein degradation in cells under favorable growth conditions. When yeast cells are under nutritional stress, most proteasomes exit the nucleus and either accumulate in cytoplasmic condensates called proteasome storage granules (PSGs) or are directed to the vacuole by autophagy. Nitrogen starvation does not cause PSG formation but leads to degradation of proteasomes through the classical macroautophagy pathway. By contrast, carbon starvation or extended incubation in stationary phase results in both PSG formation and macroautophagy of proteasomes. Unexpectedly, we found that glucose limitation also causes proteasomes to be taken up directly into vacuoles by a microautophagy mechanism. Macro- and micro-autophagy occur in parallel in glucose-starved cells, and microautophagy appears biased toward aberrant or inactive proteasomes, leaving functional proteasomes to accumulate in PSGs. PSGs dissolve and proteasomes remobilize to the nucleus within minutes after glucose refeeding. We showed that AMP-activated protein kinase (AMPK) and endosomal-sorting-complex-required-for-transport (ESCRT) factors are required for proteasome microautophagy and also impact PSG dissipation and nuclear reimport of proteasomes after glucose refeeding. The insoluble protein deposit (IPOD) compartment provides an alternative means of proteasome homeostasis, including when microautophagy is impaired. Our findings reveal a surprising diversity of mechanisms for proteasome quality and quantity control during starvation. A mechanistic understanding of the AMPK-regulated ESCRT-mediated microautophagy pathway could provide new avenues for manipulating proteasome homeostasis and treating human disease.

蛋白酶体是高度丰富的蛋白质复合物，在有利的生长条件下负责细胞中大多数受调节的蛋白质降解。当酵母细胞处于营养压力下时，大多数蛋白酶体会离开细胞核并在称为蛋白酶体储存颗粒 (PSG) 的细胞质凝聚物中积累，或者通过自噬引导至液泡。氮饥饿不会导致 PSG 形成，但会导致蛋白酶体通过经典的巨自噬途径降解。相比之下，固定相中的碳饥饿或延长孵育会导致蛋白酶体的 PSG 形成和巨自噬。出乎意料的是，我们发现葡萄糖限制也会导致蛋白酶体通过微自噬机制直接被吸收到液泡中。巨自噬和微自噬在葡萄糖饥饿的细胞中并行发生，微自噬似乎偏向于异常或无活性的蛋白酶体，使功能性蛋白酶体在 PSG 中积累。PSGs 溶解，蛋白酶体在重新补充葡萄糖后几分钟内重新进入细胞核。我们表明，蛋白酶体微自噬需要 AMP 活化蛋白激酶 (AMPK) 和内体分选复合物需要转运 (ESCRT) 因子，并且还影响葡萄糖再喂养后蛋白酶体的 PSG 耗散和核重新导入。不溶性蛋白质沉积物 (IPOD) 隔室提供了蛋白酶体稳态的替代方法，包括微自噬受损时。我们的发现揭示了饥饿期间蛋白酶体质量和数量控制的惊人多样性机制。