Gamma-tubulin and the C-terminal motor domain kinesin-like protein, KLPA, function in the establishment of spindle bipolarity in Aspergillus nidulans

Previous research has found that a gamma-tubulin mutation in Schizosaccharomyces pombe is synthetically lethal with a deletion of the C-terminal motor domain kinesin-like protein gene pkl1, but the lethality of the double mutant prevents a phenotypic analysis of the synthetic interaction. We have investigated interactions between klpA1, a deletion of an Aspergillus nidulans homolog of pkl1, and mutations in the mipA, gamma-tubulin gene. We find that klpA1 dramatically increases the cold sensitivity and slightly reduces the growth rate at all temperatures, of three mipA alleles. In synchronized cells we find that klpA1 causes a substantial but transient inhibition of the establishment of spindle bipolarity. At a restrictive temperature, mipAD123 causes a slight, transient inhibition of spindle bipolarity and a more significant inhibition of anaphase A. In the mipAD123/klpA1 strain, formation of bipolar spindles is more strongly inhibited than in the klpA1 single mutant and many spindles apparently never become bipolar. These results indicate, surprisingly, that gamma-tubulin and the klpA kinesin have overlapping roles in the establishment of spindle bipolarity. We propose a model to account for these data.

Alanine-scanning mutagenesis of Aspergillus gamma-tubulin yields diverse and novel phenotypes

We have created 41 clustered charged-to-alanine scanning mutations of the mipA, gamma-tubulin, gene of Aspergillus nidulans and have created strains carrying these mutations by two-step gene replacement and by a new procedure, heterokaryon gene replacement. Most mutant alleles confer a wild-type phenotype, but others are lethal or conditionally lethal. The conditionally lethal alleles exhibit a variety of phenotypes under restrictive conditions. Most have robust but highly abnormal mitotic spindles and some have abnormal cytoplasmic microtubule arrays. Two alleles appear to have reduced amounts of gamma-tubulin at the spindle pole bodies and nucleation of spindle microtubule assembly may be partially inhibited. One allele inhibits germ tube formation. The cold sensitivity of two alleles is strongly suppressed by the antimicrotubule agents benomyl and nocodazole and a third allele is essentially dependent on these compounds for growth. Together our data indicate that gamma-tubulin probably carries out functions essential to mitosis and organization of cytoplasmic microtubules in addition to its well-documented role in microtubule nucleation. We have also placed our mutations on a model of the structure of gamma-tubulin and these data give a good initial indication of the functionally important regions of the molecule.

Differential expression of two gamma-tubulin isoforms during gametogenesis and development in Drosophila

Previous work identified a gamma-tubulin gene, gamma Tub23C, in Drosophila (Zheng et al., 1991). We now report identification of a second gamma-tubulin gene, gamma Tub37CD. Immunoblot analysis and immunolocalization show that gamma Tub37CD and gamma Tub23C are differentially expressed during gametogenesis and development. During oogenesis, gamma Tub23C was detected at centrosomes and in the cytoplasm of mitotic germ cells, but was not detected in germ cells following completion of mitosis. Conversely, gamma Tub37CD was not detected in proliferating germ cells, but appeared to accumulate in germ cells during egg chamber development. Neither gamma-tubulin isoform was detected at the anterior or posterior poles of developing oocytes. During spermatogenesis, only gamma Tub23C was detected at centrosomes, where it showed cell cycle- and differentiation-dependent organization. During the transition into the first meiotic division, gamma Tub23C became organized as a corpuscular focus at centrioles until completion of meiosis II. During postmeiotic spermatid differentiation, gamma Tub23C was detected first as a rod and then as a collar-like structure near the juncture of the nucleus and the elongating flagellum, but was not detected in bundles of mature sperm. The germline-specific CDC25 encoded by twine is required for organization of gamma Tub23C into corpuscular focus in spermatocytes, but not for separation of centriole pairs in M-phase or postmeiotic organization of gamma Tub23C at centrioles. Following reconstitution of a canonical centrosome at fertilization, only gamma Tub37CD was detected at centrosomes in syncytial embryos, but both gamma Tub37CD and gamma Tub23C were detected at centrosomes in cellularized embryos. Colocalization of these two isoforms suggests that gamma Tub23C and gamma Tub37CD both contain structural features of gamma-tubulins essential for localization to centrosomes.

Gamma-tubulin is a component of the spindle pole body that is essential for microtubule function in Aspergillus nidulans

We have recently discovered that the mipA gene of A. nidulans encodes gamma-tubulin, a new member of the tubulin superfamily. To determine the function of gamma-tubulin in vivo, we have created a mutation in the mipA gene by integrative transformation, maintained the mutation in a heterokaryon, and determined the phenotype of the mutation in spores produced by the heterokaryon. The mutation is lethal and recessive. It strongly inhibits nuclear division, less strongly inhibits nuclear migration, and, as judged by immunofluorescence microscopy, causes a reduction in the number and length of cytoplasmic microtubules and virtually a complete absence of mitotic apparatus. We conclude that gamma-tubulin is essential for microtubule function in general and nuclear division in particular. Immunofluorescence microscopy of wild-type hyphae with affinity-purified, gamma-tubulin-specific antibodies reveals that gamma-tubulin is a component of interphase and mitotic spindle pole bodies. We propose that gamma-tubulin attaches microtubules to the spindle pole body, nucleates microtubule assembly, and establishes microtubule polarity in vivo.

Identification of gamma-tubulin, a new member of the tubulin superfamily encoded by mipA gene of Aspergillus nidulans

Microtubules, which are essential for mitosis and many other cytoskeletal functions, are composed primarily of alpha- and beta-tubulin. The properties of microtubules are due, in part, to proteins other than tubulins that are part of, or interact with, microtubules and the identification and characterization of such proteins is important to understanding how microtubules function. Analyses of mutations at the mipA (microtubule interacting protein) locus of Aspergillus nidulans have suggested that the product of mipA interacts specifically, probably physically, with beta-tubulin in vivo and is involved in microtubule function. We have cloned and sequenced the wild-type mipA gene as well as complementary DNA copies of its messenger RNA. Comparisons of the predicted product of mipA with tubulins from diverse organisms reveal that mipA is a previously undiscovered member of the tubulin superfamily of genes; the only member yet discovered that does not encode alpha- or beta-tubulin. We propose that the product of mipA be called gamma-tubulin.

Conditionally lethal tubA alpha-tubulin mutations in Aspergillus nidulans

We have mapped 17 extragenic suppressors of benA33, a heat-sensitive beta-tubulin mutation of Aspergillus nidulans, to the tubA alpha tubulin locus. Fifteen of these tubA mutations cause cold sensitivity in a genetic background with benA33 and appear to cause lethality in a background with the wild-type benA allele. We examined the microtubule-mediated processes, nuclear division and nuclear migration, in seven different cold-sensitive double mutants, each carrying benA33 and a different cold-sensitive tubA allele. Nuclear division and migration were inhibited at a restrictive temperature in each case, suggesting that cold sensitivity is due to the inhibition of microtubule function at low temperatures. A single allele, tubA4, suppressed the heat sensitivity conferred by benA33 but did not confer cold sensitivity in a benA33 background, however in a wild-type benA background, tubA4 conferred supersensitivity to antimicrotubule agents and weak cold sensitivity. TubA4 did not suppress the heat sensitivity conferred by two other benA alleles. The cold sensitivity conferred by tubA4 was suppressed by the microtubule stabilizing agent deuterium oxide, and the suppression of heat sensitivity conferred by four other tubA mutations was reversed by deuterium oxide. These results suggest that these mutations may affect hydrophobic interactions between alpha- and beta-tubulin.

Cloning of the riboB locus of Aspergillus nidulans

We have complemented the riboB2 mutation of Aspergillus nidulans by transformation with a plasmid library of wild-type (wt) sequences. We have isolated, by marker rescue from a riboB+ transformant, a plasmid that complements riboB2 efficiently. From this plasmid we have subcloned an A. nidulans sequence that complements riboB2 efficiently and that integrates by homologous recombination at a site closely linked to the riboB locus. We conclude that this sequence contains the wt riboB+ allele.

Cloning, mapping and molecular analysis of the pyrG (orotidine-5'-phosphate decarboxylase) gene of Aspergillus nidulans

We have modified the transformation procedures of Ballance et al. [Biochem. Biophys. Res. Commun. 112 (1983) 284-289] to give increased rates of transformation in Aspergillus nidulans. With the modified procedures we have been able to complement pyrG89, a mutation in the orotidine-5'-phosphate decarboxylase gene of A. nidulans, by transformation with a library of wild-type (wt) sequences in pBR329. We have recovered, by marker rescue from one such transformant, a plasmid (pJR15) that carries an A. nidulans sequence that complements pyrG89 efficiently. In three experiments, this plasmid gave an average of 1985 stable transformants/micrograms of transforming DNA. We have analyzed ten of these genetically and by Southern hybridization. In five transformants a single copy of the transforming plasmid had integrated at the pyrG locus, in one transformant several copies of pJR15 had integrated at this locus, in one transformant several copies of the plasmid had integrated into other sites, and in three transformants, the wt allele had apparently replaced the mutant allele with no integration of pBR329 sequences. Sequence and S1 nuclease protection analysis revealed that pJR15 contains a gene that predicts an amino acid sequence with regions of strong homology to the orotidine-5'-phosphate decarboxylases of Neurospora crassa and Saccharomyces cerevisiae. We conclude that this gene is the wt pyrG allele. Finally, we have compared the 5'- and 3'-noncoding sequences and intron splice sequences to other genes of A. nidulans and have mapped the pyrG locus to a region between the fpaB and galD loci on linkage group I.

Isolation of mip (microtubule-interacting protein) mutations of Aspergillus nidulans

We identified four mutations in two previously undescribed loci involved in microtubule function in Aspergillus nidulans as extragenic suppressors of benA33, a heat-sensitive beta-tubulin mutation. Three of the four mutations map to a locus closely linked to riboB on linkage group VIII; we designated this locus mipA (for microtubule-interacting protein). We were not able to map the remaining suppressor because of chromosomal rearrangements. However, since it recombines with riboB at a significantly higher frequency than the mipA alleles, it is unlikely to be in mipA; thus, we designated it mipB1. The mip mutations are not allelic to the previously identified loci that encode alpha- and beta-tubulin, and it is likely that mipA and mipB encode previously unidentified nontubulin proteins involved in microtubule function. Each of the mip mutations suppresses the heat sensitivity conferred by benA33 and suppresses the blockage of nuclear division and movement conferred by this mutation at high temperatures. Interactions between mipA and benA are allele specific. All of the mipA mutations are cryptic in a wild-type benA background but cause cold sensitivity in combination with benA33. These mutations also confer cold sensitivity in combination with benA31 and benA32 and reduce the resistance conferred by these mutations to the antimicrotubule agent benomyl but do not suppress the heat sensitivity conferred by these alleles. Finally, the mipA alleles suppress the heat sensitivity conferred by benA11, benA17, and benA21 but do not confer cold sensitivity in combination with these alleles.

Isolation and characterization of cold-sensitive mutations at the benA, beta-tubulin, locus of Aspergillus nidulans

We have isolated large numbers of conditionally lethal beta-tubulin mutations to provide raw material for analyzing the structure and function of beta tubulin and of microtubules. We have isolated such mutations as intragenic suppressors of benA33, a heat-sensitive (hs-) beta-tubulin mutation of Aspergillus nidulans. Among over 2,600 revertants isolated, 126 were cold-sensitive (cs-). In 41 of 78 cs- revertants analyzed, cold sensitivity and reversion from hs- to hs+ were due to mutations linked to benA33. In three cases reversion was due to mutations closely linked to benA33 but cold sensitivity was due to a coincidental mutation unlinked to benA33. In the remaining 34 cases reversion was due to mutations unlinked to benA33. Thirty-three of the revertants in which cold sensitivity and reversion were linked to benA33 were sufficiently cold-sensitive to allow us to select for rare recombinants between benA33 and putative suppressors in a revertant X wild-type (wt) cross. We found only one recombinant among 1,000 or more viable progeny from crosses of each of these revertants with a wt strain. Reversion is thus due to a back mutation or very closely linked suppressor in each case. We have analyzed 17 of these 33 revertants with greater precision and have found that, in each case, reversion is due to a suppressor mutation that maps to the right of benA33. The recombination frequencies between benA33 and the suppressors are very low (less than 1.2 X 10(-4)) in all cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that p-fluorophenylalanine has a direct effect on tubulin in Aspergillus nidulans

Three temperature-sensitive alleles of benA (benA11, 17 and 21) confer resistance to growth inhibition by p-fluorophenylalanine (FPA). FPA resistance cosegregates with the benA gene. Two back-mutations in benA which cause loss of temperature sensitivity cause loss of FPA resistance, and two indirect suppressors of benA temperature sensitivity also cause FPA resistance to be lost. These results indicate that FPA resistance is an intrinsic property of the benA mutations. The intracellular phenylalanine concentrations of these strains are normal as is their ability to take up phenylalanine from the medium. We conclude that FPA must inhibit growth and cause non-disjunction by a direct effect on the polymerization of tubulin.