Chapter 3 - The Nucleus-Vacuole Junction in Saccharomyces cerevisiae

Abstract

Membrane contact sites (MCSs) are formed by membranes of two closely apposed organelles (10–30 nanometers). They are observed from yeast to mammals and are assumed to allow the diffusion of ions (calcium) and lipids from one cellular compartment to another. Several MCSs have been identified in the yeast Saccharomyces cerevisiae. They link the endoplasmic reticulum on one side and a second organelle on the other. This second partner can be the plasma membrane, mitochondria, the Golgi apparatus, or endosomes. In the case of the nucleus-vacuole junction (NVJ), both membranes are bridged by an interaction between the nuclear protein Nvj1p, which spans the outer nuclear membrane, and Vac8p, which is lipid-anchored in the vacuolar membrane. NVJs are sites of a specific kind of autophagy called piecemeal microautophagy of the nucleus (PMN), or micronucleophagy. In PMN, the NVJ forms a bulge that contains a part of the nucleus and invaginates into the lumen of the vacuole. The bulge develops into a bleb structure. Finally, a vesicle carrying nuclear material is released into the vacuolar lumen to be degraded. PMN is induced under stress conditions and upon inhibition of TOR complex 1 by rapamycin treatment or nutrient limitation. Further proteins enriched at NVJs are the SMP protein Nvj2p, the lipid transfer protein Osh1p, and the fatty acid elongase Tsc13p. This points to a potential function of NVJs in lipid metabolism. The key membrane-connecting proteins Nvj1p and Vac8p have no homologues in mammals or plants that could be identified by amino acid sequence comparison. However, MCSs between ER and late endosomes/lysosomes are present in mammalian cells and other proteins enriched in NVJs are conserved in mammals and plants.