Nucleus-vacuole junctions in Saccharomyces cerevisiae are formed through the direct interaction of Vac8p with Nvj1p

Vac8p is a vacuolar membrane protein that is required for efficient vacuole inheritance and fusion, cytosol-to-vacuole targeting, and sporulation. By analogy to other armadillo domain proteins, including beta-catenin and impo rtin alpha, we hypothesize that Vac8p docks various factors at the vacuole membrane. Two-hybrid and copurfication assays demonstrated that Vac8p does form complexes with multiple binding partners, including Apg13p, Vab2p, and Nvj1p. Here we describe the surprising role of Vac8p-Nvj1p complexes in th e formation of nucleus-vacuole (NV) junctions. Nvj1p is an integral membran e protein of the nuclear envelope and interacts with Vac8p in the cytosol t hrough its C-terminal 40-60 amino acids (aa). Nvj1p green fluorescent prote in (GFP) concentrated in small patches or rafts at sites of close contact b etween the nucleus and one or more vacuoles. Previously we showed that Vac8 p-GFP concentrated in intervacuole rafts, where is it likely to facilitate vacuole-vacuole fusion, and in "orphan" rafts at the edges of vacuole clust ers. Orphan rafts of Vac8p red-sifted GFP (YFP) colocalize at sites of NV j unctions with Nvj1p blue-sifted GFP (CFP). GFP-tagged nuclear pore complexe s (NPCs) were excluded from NV junctions. In vac8-Delta cells, Nvj1p-GFP ge nerally failed to concentrate into rafts and, instead, encircled the nucleu s. NV junctions were absent in both nvj1-Delta and vac8-Delta cells. Overex pression of Nvj1p caused the profound proliferation of NV junctions. We con clude that Vac8p and Nvj1p are necessary components of a novel interorganel le junction apparatus.