A homolog of mammalian, voltage-gated calcium channels mediates yeast pheromone-stimulated Ca2+ uptake and exacerbates the cdc1(Ts) growth defect

Inositol hexakisphosphate in Schizosaccharomyces pombe: synthesis from Ins(1,4,5)P3 and osmotic regulation

Schizosaccharomyces pombe extracts synthesize InsP6 (myo-inositol hexaphosphate) from Ins(1,4,5)P3 plus ATP. An S. pombe soluble fraction converts Ins(1,4,5)P3 into Ins(1,4,5,6)P4 and Ins(1,3,4, 5)P4, in a constant ratio of approximately 5:1, and thence to Ins(1, 3,4,5,6)P5 and InsP6. We have purified a soluble Mg2+-dependent kinase of molecular mass approximately 41 kDa that makes Ins(1,4,5, 6)P4 and Ins(1,3,4,5)P4 in the same ratio and also converts Ins(1,4, 5,6)P4 or Ins(1,3,4,5)P4 into Ins(1,3,4,5,6)P5 and InsP6. Of InsP3 isomers other than Ins(1,4,5)P3, only the non-biological molecule Ins(1,4,6)P3 potently 'competed' with all steps in conversion of Ins(1,4,5)P3 into InsP6. Examination of molecular graphics representations allowed us to draw tentative conclusions about the environment needed for an hydroxyl group to be phosphorylated by this kinase and to predict successfully that the purified kinase would phosphorylate the 5-hydroxyl of Ins(1,4,6)P3. S. pombe that have been cultured with [3H]inositol contains a variety of 3H-labelled inositol polyphosphates, with Ins(1,4,5)P3 and InsP6 the most prominent, and the InsP6 concentration quickly increases in hyper-osmotically stressed S. pombe. This yeast therefore contains InsP6 and Ins(1,4,5)P3 as normal constituents, makes more InsP6 when hyper-osmotically stressed and contains a versatile inositol polyphosphate kinase that synthesizes InsP6 from Ins(1,4,5)P3.

NSC1: a novel high-current inward rectifier for cations in the plasma membrane of Saccharomyces cerevisiae

The plasma membrane of the yeast Saccharomyces cerevisiae possesses a non-specific cation 'channel', tentatively dubbed NSC1, which is blocked by normal (mM) calcium and other divalent metal ions, is unblocked by reduction of extracellular free divalents below approximately 10 microM, and is independent of the identified potassium channel and porters in yeast, Duk1p, Trk1p, and Trk2p. Ion currents through NSC1, observed by means of whole-cell patch recording, have the following characteristics: Large amplitude, often exceeding 1 nA of K+/ cell at -200 mV, in tetraploid yeast, sufficient to double the normal intracellular K+ concentration within 10 s; non-saturation at large negative voltages; complicated activation kinetics, in which approximately 50% of the total current arises nearly instantaneously with a voltage-clamp step, while the remainder develops as two components, with time constants of approximately 100 ms and approximately 1.3 s; and voltage independence of both the activation time constants and the associated fractional current amplitudes.

Mapping fungal ion channel locations

Ion channel mapping techniques are described and the results for two fungal organisms, Saprolegnia ferax and Neurospora crassa, are presented. In these species, two channel types have been characterized, stretch-activated channels exhibiting significant calcium permeability and spontaneous channels having significant potassium permeability. Two distinct analyses of patch clamp data, analysis of channel self-clustering and association between different channel types, and localization along the hyphae, reveal significant differences between the two organisms. S. ferax maintains a tip-high gradient of both channel types which is lost after disruption of the actin cytoskeleton. There is significant self-clustering of the channels, as well as interactions between channel types. N. crassa on the other hand does not maintain tip-high gradients, and clustered distributions are observed only for the stretch-activated channels. In terms of physiological roles, evidence is quite strong that the stretch-activated channels function as a growth sensor in S. ferax, but have an unknown function in N. crassa. In both organisms, the potassium permeable channels presumably function in potassium uptake. The differences between these two organisms may be due, in part, to differences in their normal environment: aquatic versus terrestrial. Copyright 1998 Academic Press.

INP51, a yeast inositol polyphosphate 5-phosphatase required for phosphatidylinositol 4,5-bisphosphate homeostasis and whose absence confers a cold-resistant phenotype

Sequence analysis of Saccharomyces cerevisiae chromosome IX identified a 946 amino acid open reading frame (YIL002C), designated here as INP51, that has carboxyl- and amino-terminal regions similar to mammalian inositol polyphosphate 5-phosphatases and to yeast SAC1. This two-domain primary structure resembles the mammalian 5-phosphatase, synaptojanin. We report that Inp51p is associated with a particulate fraction and that recombinant Inp51p exhibits intrinsic phosphatidylinositol 4,5-bisphosphate 5-phosphatase activity. Deletion of INP51 (inp51) results in a "cold-tolerant" phenotype, enabling significantly faster growth at temperatures below 15 degreesC as compared with a parental strain. Complementation analysis of an inp51 mutant strain demonstrates that the cold tolerance is strictly due to loss of 5-phosphatase catalytic activity. Furthermore, deletion of PLC1 in an inp51 mutant does not abrogate cold tolerance, indicating that Plc1p-mediated production of soluble inositol phosphates is not required. Cells lacking INP51 have a 2-4-fold increase in levels of phosphatidylinositol 4,5-bisphosphate and inositol 1,4, 5-trisphosphate, whereas cells overexpressing Inp51p exhibit a 35% decrease in levels of phosphatidylinositol 4,5-bisphosphate. We conclude that INP51 function is critical for proper phosphatidylinositol 4,5-bisphosphate homeostasis. In addition, we define a novel role for a 5-phosphatase loss of function mutant that improves the growth of cells at colder temperatures without alteration of growth at normal temperatures, which may have useful commercial applications.