Fast-growing Arctic Fe-Mn deposits from the Kara Sea as the refuges for cosmopolitan marine microorganisms

The impact of biomineralization and redox processes on the formation and growth of ferromanganese deposits in the World Ocean remains understudied. This problem is particularly relevant for the Arctic marine environment where sharp seasonal variations of temperature, redox conditions, and organic matter inflow significantly impact the biogenic and abiotic pathways of ferromanganese deposits formation. The microbial communities of the fast-growing Arctic Fe-Mn deposits have not been reported so far. Here, we describe the microbial diversity, structure and chemical composition of nodules, crust and their underlying sediments collected from three different sites of the Kara Sea. Scanning electron microscopy revealed a high abundance of microfossils and biofilm-like structures within the nodules. Phylogenetic profiling together with redundancy and correlation analyses revealed a positive selection for putative metal-reducers (Thermodesulfobacteriota), iron oxidizers (Hyphomicrobiaceae and Scalinduaceae), and Fe-scavenging Nitrosopumilaceae or Magnetospiraceae in the microenvironments of the Fe-Mn deposits from their surrounding benthic microbial populations. We hypothesize that in the Kara Sea, the nodules provide unique redox-stable microniches for cosmopolitan benthic marine metal-cycling microorganisms in an unsteady environment, thus focusing the overall geochemical activity of nodule-associated microbial communities and accelerating processes of ferromanganese deposits formation to uniquely high rates.

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 importin 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 the formation of nucleus-vacuole (NV) junctions. Nvj1p is an integral membrane protein of the nuclear envelope and interacts with Vac8p in the cytosol through its C-terminal 40-60 amino acids (aa). Nvj1p green fluorescent protein (GFP) concentrated in small patches or rafts at sites of close contact between the nucleus and one or more vacuoles. Previously, we showed that Vac8p-GFP concentrated in intervacuole rafts, where is it likely to facilitate vacuole-vacuole fusion, and in "orphan" rafts at the edges of vacuole clusters. Orphan rafts of Vac8p red-sifted GFP (YFP) colocalize at sites of NV junctions with Nvj1p blue-sifted GFP (CFP). GFP-tagged nuclear pore complexes (NPCs) were excluded from NV junctions. In vac8-Delta cells, Nvj1p-GFP generally failed to concentrate into rafts and, instead, encircled the nucleus. NV junctions were absent in both nvj1-Delta and vac8-Delta cells. Overexpression of Nvj1p caused the profound proliferation of NV junctions. We conclude that Vac8p and Nvj1p are necessary components of a novel interorganelle junction apparatus.

A nuclear export signal prevents Saccharomyces cerevisiae Hsp70 Ssb1p from stimulating nuclear localization signal-directed nuclear transport

Hsp70 has been implicated in nuclear localization signal (NLS)-directed nuclear transport. Saccharomyces cerevisiae contains distinct SSA and SSB gene families of cytosolic Hsp70s. The nucleocytoplasmic localization of Ssa1p and Ssb1p was investigated using green fluorescent protein (GFP) fusions. Whereas GFP-Ssa1p localized both to the nucleus and cytoplasm, GFP-Ssb1p appeared only in the cytosol. The C-terminal domain of Ssb1p contains a leucine-rich nuclear export signal (NES) that is necessary and sufficient to direct nuclear export. The accumulation of GFP-Ssb1p in the nuclei of xpo1-1 cells suggests that Ssb1p shuttles across the nuclear envelope. Elevated levels of SSA1 but not SSB1 suppressed the NLS-GFP nuclear localization defects of nup188-Delta cells. Studies with Ssa1p/Ssb1p chimeras revealed that the Ssb1p NES is sufficient and necessary to inhibit the function of Ssa- or Ssb-type Hsp70s in nuclear transport. Thus, NES-less Ssb1p stimulates nuclear transport in nup188-Delta cells and NES-containing Ssa1p does not. We conclude that the differential function of Ssa1p and Ssb1p in nuclear transport is due to the NES-directed export of the Ssb1p and not to functional differences in their ATPase or peptide binding domains.

In vivo nuclear transport kinetics in Saccharomyces cerevisiae: a role for heat shock protein 70 during targeting and translocation

The transport of proteins into the nucleus is a receptor-mediated process that is likely to involve between 50-100 gene products, including many that comprise the nuclear pore complex. We have developed an assay in Saccharomyces cerevisiae for the nuclear transport of green fluorescent protein fused to the SV-40 large T antigen nuclear localization signal (NLS-GFP). This assay allows the measurement of relative NLS-GFP nuclear import rates in wild-type and mutant cells under various physiological conditions. Probably the best understood component of the nuclear transport apparatus is Srp1p, the NLS receptor, which binds NLS-cargo in the cytoplasm and accompanies it into the nucleus. When compared to SRP1+ cells, NLS-GFP import rates in temperature-sensitive srp1-31 cells were slower and showed a lower temperature optimum. The in vivo transport defect of the srp1-31 cells was correlated with the purified protein's thermal sensitivity, as assayed by in vitro NLS peptide binding. We show that the kinetics of NLS-directed nuclear transport in wild-type cells is stimulated by the elevated expression of SSA1, which encodes a cytoplasmic heat shock protein 70 (Hsp70). Elevated Hsp70 levels are sufficient to suppress the NLS-GFP import defects in srp1-31 and nup82-3 cells. NUP82 encodes a protein that functions within the nuclear pore complex subsequent to docking. These results provide genetic evidence that Hsp70 acts during both targeting and translocation phases of nuclear transport, possibly as a molecular chaperone to promote the formation and stability of the Srp1p-NLS-cargo complex.