Piecemeal Microautophagy of the Nucleus Requires the Core Macroautophagy Genes

Abstract

Autophagy is a diverse family of processes that transport cytoplasm and organelles into the lysosome/vacuole lumen for degradation. During macroautophagy cargo is packaged in autophagosomes that fuse with the lysosome/vacuole. During microautophagy cargo is directly engulfed by the lysosome/vacuole membrane. Piecemeal microautophagy of the nucleus (PMN) occurs in Saccharomyces cerevisiae at nucleus-vacuole (NV) junctions and results in the pinching-off and release into the vacuole of nonessential portions of the nucleus. Previous studies concluded macroautophagy ATG genes are not absolutely required for PMN. Here we report using two biochemical assays that PMN is efficiently inhibited in atg mutant cells: PMN blebs are produced, but vesicles are rarely released into the vacuole lumen. Electron microscopy of arrested PMN structures in atg7, atg8, and atg9 mutant cells suggests that NV-junction-associated micronuclei may normally be released from the nucleus before their complete enclosure by the vacuole membrane. In this regard PMN is similar to the microautophagy of peroxisomes (micropexophagy), where the side of the peroxisome opposite the engulfing vacuole is capped by a structure called the "micropexophagy-specific membrane apparatus" (MIPA). The MIPA contains Atg proteins and facilitates terminal enclosure and fusion steps. PMN does not require the complete vacuole homotypic fusion genes. We conclude that a spectrum of ATG genes is required for the terminal vacuole enclosure and fusion stages of PMN.

Autophagy and genomic integrity

“…63 Autophagy of nuclear components in eukaryotes, or piecemeal microautophagy of the nucleus (PMN), was initially described in Saccharomyces cerevisae. It is triggered by nutrient deprivation and occurs through the release of nuclear portions into the vacuole, 64 followed by digestion by hydrolases. 65 Nuclear components targeted for PMN include granular nucleolus enriched in pre-ribosomes and nuclear envelope, nuclear pore complexes or spindle pole bodies.…”

“…66,67 PMN involves the core Atg proteins involved in macroautophagy and microautophagy, such as Atg4, 5, 7 and 12, as well as macroautophagy-specific proteins such as Atg17, 29 and 31. 64 In mammalian cells, autophagy can degrade nuclear components, thereby contributing to the maintenance of nuclear function and integrity. 68 The initial observation of the engulfment of nuclear components points to the presence of perinuclear vacuoles in skeletal and/or cardiac muscle cells from patients or mice with envelopathies, 68 disorders caused by mutations in nuclear envelope components.…”

The role of ALFY in selective autophagy

“…49 If and how nuclear aggregates are degraded by autophagy is currently not known, but three different models can be proposed; (i) p62 and ALFY bind and export misfolded ubiquitinated proteins through the nuclear pores, (ii) p62 and ALFY associate to nuclear aggregates that become degraded by autophagy during mitosis when the nuclear membrane breaks down and (iii) p62 and ALFY facilitate sorting of nuclear aggregates into nuclear membrane blebs, which become pinched off and degraded by autophagy in a process similar to yeast piecemeal microautophagy of the nucleus. 70 The finding that the nuclear oncoprotein PML/RARA, expressed in acute promyelocytic leukemia (APL) patients from a chromosomal t(15,17) translocation, is degraded by autophagy in a p62-dependent manner strongly indicate that nuclear aggregate-prone proteins can be targeted by selective autophagy. 71 It will be exciting to learn if ALFY is also involved in PML/RARA degradation and if/how p62 and/or ALFY facilitate recruitment of PML/RARA from the nucleus to the cytoplasm for degradation by autophagy.…”

Structural and functional characterization of the two phosphoinositide binding sites of PROPPINs, a β-propeller protein family

“…Atg18 also binds PtdIns(3,5)P 2 -dependent to the vacuole, where it carries out nonautophagic functions, such as the regulation of the PtdIns3P 5-kinase Fab1, and vesicular transport from the vacuole to the Golgi (2,19,20). Atg21 typically functions in selective autophagy variants (4,21,22). The function of Hsv2 is unclear; only its requirement for efficient PMN has been described (21).…”