In order to identify additional factors required for nuclear export of messenger RNA, a genetic screen was conducted with a yeast mutant deficient in a factor Gle1p, which associates with the nuclear pore complex (NPC). The three genes identified encode phospholipase C and two potential inositol polyphosphate kinases. Together, these constitute a signaling pathway from phosphatidylinositol 4, 5-bisphosphate to inositol hexakisphosphate (IP6). The common downstream effects of mutations in each component were deficiencies in IP6 synthesis and messenger RNA export, indicating a role for IP6 in GLE1 function and messenger RNA export.

Synthesis of inositol 1,2,3,4,5,6-hexakisphosphate (IP 6 ), also known as phytate, is integral to cellular function in all eukaryotes. Production of IP 6 predominately occurs through phosphorylation of inositol 1,3,4,5,6-pentakisphosphate (IP 5 ) by a 2-kinase. Recent cloning of the gene encoding this kinase from Saccharomyces cerevisiae, designated scIpk1, has identified a cellular role for IP 6 production in the regulation of mRNA export from the nucleus. In this report, we characterize the biochemical and functional parameters of recombinant scIpk1. Purified recombinant scIpk1 kinase activity is highly selective for IP 5 substrate and exhibits apparent K m values of 644 nM and 62.8 M for IP 5 and ATP, respectively. The observed apparent catalytic efficiency (k cat / K m ) of scIpk1 is 31,610 s ؊1 M ؊1 . A sequence similarity search was used to identify an IP 5 2-kinase from the fission yeast Schizosaccharomyces pombe. Recombinant spIpk1 has similar substrate selectivity and catalytic efficiency to its budding yeast counterpart, despite sharing only 24% sequence identity. Cells lacking sc-IPK1 are deficient in IP 6 production and exhibit lethality in combination with a gle1 mutant allele. Both of these phenotypes are complemented by expression of the spIPK1 gene in the sc-ipk1 cells. Analysis of several inactive mutants and multiple sequence alignment of scIpk1, spIpk1, and a putative Candida albicans Ipk1 have identified residues involved in catalysis. This includes two conserved motifs: E(i/l/m)KPKWL(t/y) and LXMTLRDV(t/g)(l/c)(f/y)I. Our data suggest that the mechanism for IP 6 production is conserved across species.Inositol polyphosphates (IPs) 1 in eukaryotic cells are key regulatory molecules whose levels transiently fluctuate in response to diverse cellular stimuli (1, 2). A major route for synthesis of IPs is through activation of phosphatidylinositolspecific phospholipase C. Phospholipase C cleaves lipids such as phosphatidylinositol 4,5-bisphosphate to generate inositol 1,4,5-trisphosphate, a regulator of calcium efflux from the endoplasmic reticulum. The release of a soluble inositol head group from its anchoring lipid also represents the first step in the pathway for generation of more highly phosphorylated inositols (3). The most abundant of these is inositol 1,2,3,4,5,6-hexakisphosphate (IP 6 ), also known as phytate. IP 6 can represent up to 1% of the mass of a plant seed, where it may serve as an antioxidant and a phosphate storage source (4, 5). The role of IP 6 is less clear in mammalian cells, although there is evidence suggesting that it may regulate inflammation, neurotransmission, and cell growth (reviewed in Ref.3). Recently, a metabolic pathway converting inositol 1,4,5-trisphosphate to IP 6 was delineated in budding yeast Saccharomyces cerevisiae cells (6-9). It has been shown that IP 6 also serves as a precursor for diphosphorylated inositols, such as diphosphoryl inositol 1,3,4,5,6-pentakisphosphate (PP-IP 5 ), in both yeast and vertebrate cells (3, 10). Combined in vivo and in vitro...

Integral membrane proteins are predicted to play key roles in the biogenesis and function of nuclear pore complexes (NPCs). Revealing how the transport apparatus is assembled will be critical for understanding the mechanism of nucleocytoplasmic transport. We observed that expression of the carboxyl-terminal 200 amino acids of the nucleoporin Nup116p had no effect on wild-type yeast cells, but it rendered the nup116 null strain inviable at all temperatures and coincidentally resulted in the formation of nuclear membrane herniations at 23 degrees C. To identify factors related to NPC function, a genetic screen for high-copy suppressors of this lethal nup116-C phenotype was conducted. One gene (designated SNL1 for suppressor of nup116-C lethal) was identified whose expression was necessary and sufficient for rescuing growth. Snl1p has a predicted molecular mass of 18.3 kDa, a putative transmembrane domain, and limited sequence similarity to Pom152p, the only previously identified yeast NPC-associated integral membrane protein. By both indirect immunofluorescence microscopy and subcellular fractionation studies, Snl1p was localized to both the nuclear envelope and the endoplasmic reticulum. Membrane extraction and topology assays suggested that Snl1p was an integral membrane protein, with its carboxyl-terminal region exposed to the cytosol. With regard to genetic specificity, the nup116-C lethality was also suppressed by high-copy GLE2 and NIC96. Moreover, high-copy SNL1 suppressed the temperature sensitivity of gle2-1 and nic96-G3 mutant cells. The nic96-G3 allele was identified in a synthetic lethal genetic screen with a null allele of the closely related nucleoporin nup100. Gle2p physically associated with Nup116p in vitro, and the interaction required the N-terminal region of Nup116p. Therefore, genetic links between the role of Snl1p and at least three NPC-associated proteins were established. We suggest that Snl1p plays a stabilizing role in NPC structure and function.