Abstract. The NUM1 gene is involved in the control of nuclear migration in Saccharornyces cerevisiae. The content of NUM1 mRNA fluctuates during the cell cycle, reaching a maximum at S/G2 phase, and the translation product Numlp associates with the cortex of mother cells mainly during S, G2, and mitosis, as seen by indirect immunofluorescence. The nuclear spindle in NUMl-deficient large-budded cells often fails to align along the mother/bud axis, while abnormally elongated astral microtubules emanate from both spindle pole bodies. A numl null mutation confers temperature sensitivity to the cold-sensitive a-tubulin mutant tubl-1, and shows synthetic lethality with the 13-tubulin mutant alleles tub2-402, tub2-403, tub2-404, and tub2-405. Deletion mapping has defined three functionally important Numlp regions: a potential EF hand Ca 2+ binding site, a cluster of potential phosphorylation sites and a pleckstrin homology domain. The latter domain appears to be involved in targeting Numlp to the mother cell cortex. Our data suggest that the periodically expressed NUM1 gene product controls nuclear migration by affecting astral microtubule functions.

Num1p, a cortical 313-kD protein, controls cytoplasmic microtubule (cMT) functions and nuclear migration through the bud neck in anaphase cells. A green fluorescent protein (GFP)-Num1p fusion protein localizes at the bud tip and the distal mother pole of living cells, apparently forming cMT capture sites at late anaphase. In addition, galactose-induced GFP-Num1p is seen at the bud neck and in lateral regions of the mother cortex. The bud tip location of Num1p depends on Bni1p but does not require Kar9p, Dyn1p, or cMTs, whereas cMT contacts with polar Num1p dots are reduced in cells lacking Dyn1p. Num1p associates with the dynein intermediate chain Pac11p in the presence of Dyn1p, and with the α-tubulin Tub3p, as shown by coimmune precipitation of tagged proteins. Num1p also forms a complex with Bni1p and Kar9p, although Num1p is not required for Bni1p- and Kar9p-dependent nuclear migration to the bud neck in preanaphase cells.Our data suggest that Num1p controls nuclear migration during late anaphase by forming dynein-interacting cortical cMT capture sites at both cellular poles. In addition, Num1p may transiently cooperate with an associated Bni1p–Kar9p complex at the bud tip of early anaphase cells.

The hormone receptor-like protein Gpr1p physically interacts with phosphatidylinositol-specific phospholipase C (Plc1p) and with the G␣ protein Gpa2p, as shown by two-hybrid assays and co-immune precipitation of epitope-tagged proteins. Plc1p binds to Gpr1p in either the presence or absence of Gpa2, whereas the Gpr1p/Gpa2p association depends on the presence of Plc1p. Genetic interactions between the null mutations plc1⌬, gpr1⌬, gpa2⌬, and ras2⌬ suggest that Plc1p acts together with Gpr1p and Gpa2p in a growth control pathway operating in parallel to the Ras2p function. Diploid cells lacking Gpr1p, Plc1p, or Gpa2p fail to form pseudohyphae upon nitrogen depletion, and the filamentation defect of gpr1⌬ and plc1⌬ strains is rescued by activating a mitogen-activated protein kinase pathway via STE11-4 or by activating a cAMP pathway via overexpressed Tpk2p. Plc1p is also required for efficient expression of the FG(TyA)::lacZ reporter gene under nitrogen depletion.In conclusion, we have identified two physically interacting proteins, Gpr1p and Plc1p, as novel components of a nitrogen signaling pathway controlling the developmental switch from yeast-like to pseudohyphal growth. Our data suggest that phospholipase C modulates the interaction of the putative nutrient sensor Gpr1p with the G␣ protein Gpa2p as a downstream effector of filamentation control.

We have isolated a novel gene (NUM1) with unusual internal periodicity. The NUM1 gene encodes a 313 kDa protein with a potential Ca2+ binding site and a central domain containing 12 almost identical tandem repeats of a 64 amino acid polypeptide. num1-disrupted strains grow normally, but contain many budded cells with two nuclei in the mother cell instead of a single nucleus at the bud neck, while all unbudded cells are uninucleate. This indicates that most G2 nuclei divide in the mother before migrating to the neck, followed by the migration of one of the two daughter nuclei into the bud. Furthermore, haploid num1 strains tend to diploidize during mitosis, and homozygous num1 diploid or tetraploid cells sporulate to form many budded asci with up to eight haploid or diploid spores, respectively, indicating that meiosis starts before nuclear redistribution and cytokinesis. Our data suggest that the NUM1 protein is involved in the interaction of the G2 nucleus with the bud neck.

Functional mapping of the cell cycle START gene CDC25 has revealed two domains which are dispensable for viability (germination and growth in glucose media), but are essential for sporulation and differentially involved in glucose-induced CAMP signaling. The transient rise of CAMP is completely prevented by various deletions within the amino-tenninal half (a domain) of the CDC25 gene product. In contrast, the deletion of the carboxy-terminal 38 residues (82 domain) results in a rapid, but persisting, rise of CAMP. Our data suggest that the a domain of the CDC25 protein is involved in glucose signal transduction, whereas the gZ domain is required for downregulating the CAMP control chain.