Analysis of conserved binding proteins for nuclear localization sequences.

Correct targeting of nuclear proteins is mediated by nuclear localization sequences (NLS) which permit specific binding to the nucleus and subsequent translocation across the nuclear envelope via the nuclear pore complex. It is proposed that nuclear import is facilitated by NLS-receptors which reside in the cytoplasm and at the nuclear pore. These NLS-receptors could facilitate an early step of nuclear protein import, i.e. targeting and binding of nuclear proteins at the nuclear pore. We have generated anti-idiotype antibodies against the SV40 T-antigen nuclear localization sequence that allowed us to study NLS-binding proteins in a variety of different organisms. Proteins of similar size are recognized by these antibodies in yeast, Drosophila, rat and human cells. Cytological analysis indicates that the NLS-binding proteins reside in part at nuclear pores. One of the proteins recognized by anti-idiotype antibodies is identical to a previously identified NLS-binding protein. Using isolated yeast nuclei we demonstrate that the anti-idiotype antibodies compete for binding of nuclear proteins in vitro. We show that the yeast mutant npl3, which is defective in nuclear protein localization, has an altered distribution of antigens recognized by these anti-idiotype antibodies, at the semi-permissive temperature. Our results suggest that a set of proteins common to various eukaryotes recognizes nuclear localization sequences.

Movement of macromolecules between the cytoplasm and the nucleus in yeast.

Since the first description of signals for nuclear protein localization, studies with yeast have played an important role in our understanding of nuclear protein import. Very recent experiments suggest that new insights into the poorly understood process of RNA export will also emerge from analyses of yeast. Recent advances have facilitated our understanding of protein and RNA exchange between the nucleus and cytoplasm.

A mutant nuclear protein with similarity to RNA binding proteins interferes with nuclear import in yeast.

We have isolated mutants of the yeast Saccharomyces cerevisiae that are defective in localization of nuclear proteins. Chimeric proteins containing the nuclear localization sequence from SV40 large T-antigen fused to the N-terminus of the mitochondrial F1 beta-ATPase are localized to the nucleus. Npl (nuclear protein localization) mutants were isolated by their ability to grow on glycerol as a consequence of no longer exclusively targeting SV40-F1 beta-ATPase to the nucleus. All mutants with defects in localization of nucleolar proteins and histones are temperature sensitive for growth at 36 degrees C. Seven alleles of NPL3 and single alleles of several additional genes were isolated. NPL3 mutants were studied in detail. NPL3 encodes a nuclear protein with an RNA recognition motif and similarities to a family of proteins involved in RNA metabolism. Our genetic analysis indicates that NPL3 is essential for normal cell growth; cells lacking NPL3 are temperature sensitive for growth but do not exhibit a defect in localization of nuclear proteins. Taken together, these results indicate that the mutant forms of Npl3 protein isolated by this procedure are interfering with nuclear protein uptake in a general manner.

Nutritional regulation of yeast delta-9 fatty acid desaturase activity.

The addition of unsaturated fatty acids to cultures of Saccharomyces cerevisiae significantly altered the microsomal lipid composition. Supplementation with either of the naturally occurring palmitoleic (16:1) or oleic (18:1) acids caused increased levels in membrane phospholipids and reduced levels of the complementary acid. Growth in the presence of equimolar quantities of 16:1 and 18:1 acids, however, produced a fatty acid composition similar to that found in unsupplemented cell membranes. Linoleic acid (18:2) was not found in S. cerevisiae grown under normal conditions. It was preferentially internalized and incorporated into microsomes, however, at levels exceeding 50% of the total fatty acid species. This resulted in an almost total loss of 16:1 and a reduction of 18:1 to 25% of its normal level. The delta-9 fatty acid desaturase, a microsomal enzyme that forms 16:1 and 18:1 from saturated acyl coenzyme A precursors, was affected by the presence of exogenous fatty acids. Enzyme activity toward the 16:0 coenzyme A substrate was elevated in microsomes from saturated-fatty-acid-supplemented cultures and sharply repressed following the addition of unsaturated fatty acids, including 18:2. Northern (RNA blot) and slot-blot analyses of mRNA encoded by the OLE1 gene, which appears to be the structural gene for the delta-9 desaturase, indicated that it was sharply reduced in unsaturated-fatty-acid-fed cells. These data suggest that a significant part of the regulation involves modulation of available transcripts.