ABSTRACT
Portions of the Saccharomyces cerevisiae nucleus are targeted to the vacuole and degraded by “piecemeal microautophagy of the nucleus” (Pmn). During Pmn small teardrop-like nuclear envelope blebs are engulfed by invaginations of the vacuole membrane, pinched into the vacuole lumen, and degraded by lumenal hydrolases. Pmn occurs in the context of nucleus-vacuole (NV) junctions, which are Velcro-like patches formed through specific inter actions between Vac8 on the vacuole membrane and Nvjl on the outer nuclear membrane. Pmn occurs at low levels in early log phase cells and is gradually induced as nutrients become limiting. Degradation of nonessential nuclear components such as nucleolar preribosomes ap pears to be a normal physiological process because cells undergoing Pmn survive and continue to divide. Like other autophagic processes, Pmn is induced by rapamycin, an inhibitor of Tor kinase function. Pmn occurs at normal levels in apg7lS cells, which are defective in macroautophagy. Because yeast has a closed mitosis, Pmn may have evolved to deliver damaged or excess nuclear contents to the vacuole for degradation and recycling.
