A temperature-sensitive splicing mutation in the bimG gene of Aspergillus produces an N-terminal fragment which interferes with type 1 protein phosphatase function.

Progression through anaphase requires high levels of type 1 protein phosphatase (PP1) activity in a variety of eukaryotes, including Aspergillus nidulans. A conditional lethal, temperature-sensitive mutant in one of the Aspergillus PP1 genes, bimG, prevents the normal completion of anaphase when cells are grown at restrictive temperature and this has been shown to be due to a reduction in type 1 phosphatase activity. We show that the bimG11 allele is recessive to the wild-type allele in heterozygous diploids, implying that the mutation is due to loss of function at restrictive temperature, but molecular disruption of the wild-type bimG gene shows that the gene is not essential and has no discernable phenotype under laboratory conditions. Sequence comparison of wild-type and mutant alleles reveals a single base pair difference between the two genes, within the 5' splicing site of the second intron. We demonstrate that the conditional lethal phenotype of bimG11 strains is due to impaired splicing of the mutant mRNA and that this leads to the production of a truncated protein comprising an intact N-subdomain and a modified C-terminus. Over-expression of this truncated form of PP1 in a wild-type haploid produces a lethal phenotype and reduced PP1 activity, supporting the idea that a toxic interfering protein is produced. PP1, therefore, may have at least two spatially separated sites, both of which are required for function. Temperature-sensitive splicing mutations may provide a novel means of engineering conditional versions of other proteins, particularly other phosphatases.     

PCR cloning of the cDNA of rabbit skeletal muscle protein phosphatase inhibitor-2.

The coding region of the cDNA of protein phosphatase inhibitor-2 was determined by polymerase chain reaction amplification. The cDNA clone consisted of 621 nucleotides, and encoded 204 amino acids. The deduced amino-acid sequence was identical with that of the sequence reported by chemical sequencing methods.     

Latent forms of type-1 protein phosphatase in rabbit skeletal muscle

We have examined the characteristics of partially purified forms of rabbit skeletal muscle type-1 protein phosphatase (PP-1). Over 90% of PP-1 in unfractionated extracts and in glycogen particles was inactive, but could be activated by the divalent cations, Mn2+ or Co2+ (Me2+) plus trypsin. Gel filtration of muscle extracts revealed two inactive forms of PP-1; one activated by Me2+ alone or Me2+ plus trypsin, and a second containing inhibitor-2 and activated only by Me2+ plus trypsin. The kinetics of Me2+ plus trypsin activation revealed that after DEAE-chromatography, PP-1 was altered from the forms in crude extracts, gel filtered extracts or glycogen particles. The results indicate that the purified form of PP-1 catalytic subunit has properties which distinguish it from the forms of the enzyme in muscle extracts.     

Structure of the gene encoding rabbit alpha s1-casein.

The complete nucleotide sequence of the entire rabbit alpha s1-casein-encoding gene Aslca and its flanking regions was determined. These data represent the first complete primary sequence of an Aslca gene. The gene consists of 19 exons spread over 16 kb. Highly conserved sequences were found between this gene and other casein-encoding genes mainly upstream from the gene from position -180 to -10. Several repeated interspersed elements of unknown function were also identified within introns.     

Isolation and sequence analysis of a cDNA clone encoding a type-1 protein phosphatase catalytic subunit: homology with protein phosphatase 2A

A 1.5 kb clone containing the full-length coding sequence of a type-1 protein phosphatase catalytic subunit has been isolated from a rabbit skeletal muscle cDNA library constructed in lambda gt10. The protein sequence deduced from the cDNA contains 311 residues and has a molecular mass of 35.4 kDa. A single mRNA species at 1.6 kb was visualized by Northern blotting. The type-1 protein phosphatase was strikingly homologous to protein phosphatase 2A, 49% of the amino acids between residues 11 and 280 being identical. The first 10 and last 31 residues were dissimilar. Residues 1-101 of the type-1 protein phosphatase also showed 21% sequence identity with a region of mammalian alkaline phosphatases.     

The bimG gene of Aspergillus nidulans, required for completion of anaphase, encodes a homolog of mammalian phosphoprotein phosphatase 1

In Aspergillus nidulans, the temperature-sensitive, recessive cell cycle mutation bimG11 causes an elevated mitotic index at restrictive temperature and an inability to complete the anaphase separation of daughter nuclei. We have shown that this mutation has an abnormally high content of nuclear phosphoproteins and that the wild-type gene encodes a type 1 protein phosphatase. We conclude that dephosphorylation of a key protein(s) is required to complete mitosis.     

RACH2, a novel human gene that complements a fission yeast cell cycle checkpoint mutation.

We have identified a novel human gene by virtue of its ability to complement the rad1-1 checkpoint mutant of Schizosaccharomyces pombe. This gene, called RACH2, rescues the temperature-sensitive lethality of a rad1-1 wee1-50 double mutant of S. pombe. Expression of RACH2 in S. pombe rad1-1 strains partially restores UV resistance to the rad1-1 mutant strain. Expression of RACH2 in a rad1-1 cdc25-22 double mutant partially restores the dose-dependent delay in mitotic entry after irradiation that is lost in rad1-1 checkpoint-deficient mutants. Overexpression of RACH2 in human tissue culture cells induces apoptosis.     

Identification of the phosphatase deinhibitor protein phosphatases in rabbit skeletal muscle.

In rabbit skeletal muscle the polycation-stimulated (PCS) protein phosphatases [Merlevede (1985) Adv. Protein Phosphatases 1, 1-18] are the only phosphatases displaying significant activity toward the deinhibitor protein. Among them, the PCSH protein phosphatase represents more than 80% of the measurable deinhibitor phosphatase activity associated with the PCS phosphatases. The deinhibitor phosphatase activity co-purifies with the PCSH phosphatase to apparent homogeneity. In the last purification step two forms of PCSH phosphatase were separated (PCSH1, containing 62, 55 and 34 kDa subunits, and PCSH2, containing 62 and 35 kDa subunits), both showing the same deinhibitor/phosphorylase phosphatase activity ratio. The activity of the PCSH phosphatase toward the deinhibitor is not stimulated by polycations such as protamine, histone H1 or polylysine, unlike the stimulation observed with phosphorylase as the substrate. The phosphorylase phosphatase activity of PCSH phosphatase is inhibited by ATP, PPi and Pi, whereas the deinhibitor phosphatase activity of the enzyme is much less sensitive to these agents.