Multiplicity of protein serine-threonine phosphatases in PC12 pheochromocytoma and FTO-2B hepatoma cells

p116Rip Decreases Myosin II Phosphorylation by Activating Myosin Light Chain Phosphatase and by Inactivating RhoA

p116Rip was originally found to be a RhoA-binding protein, but its function has been unknown. Here, we clarify the function of p116Rip. Two critical findings were made. First, we found that p116Rip activated the GTPase activity of RhoA in vitro and that p116Rip overexpression in cells consistently diminished the epidermal growth factor-induced increase in GTP-bound RhoA. Second, p116Rip activated the myosin light chain phosphatase (MLCP) activity of the holoenzyme. p116Rip did not activate the catalytic subunit alone, indicating that the activation is due to the binding of p116Rip to the myosin phosphatase targeting subunit MYPT1. Interestingly, the activation of phosphatase was specific to myosin as substrate, and p116Rip directly bound to myosin, thus facilitating myosin/MLCP interaction. The gene silencing of p116Rip consistently and significantly increased myosin phosphorylation as well as stress fiber formation in cells. Based upon these findings, we propose that p116Rip is an important regulatory component that controls the RhoA signaling pathway, thus regulating MLCP activity and myosin phosphorylation in cells.

Molecular cloning, expression, and characterization of a novel human serine/threonine protein phosphatase, PP7, that is homologous to Drosophila retinal degeneration C gene product (rdgC)

A novel serine/threonine protein phosphatase (PPase) designated PP7 was identified from cDNA produced from human retina RNA. PP7 has a molecular mass of approximately 75 kDa, and the deduced amino acid sequence of PP7 contains a phosphatase catalytic core domain that possesses all of the invariant motifs of the PP1, PP2A, PP2B, PP4, PP5, and PP6 gene family. However, PP7 has unique N- and C-terminal regions and shares < 35% identity with the other known PPases. The unique C-terminal region of PP7 contains multiple Ca2+ binding sites (i.e. EF-hand motifs). This region of PP7 is similar to the Drosophila retinal degeneration C gene product (rdgC), and PP7 and rdgC share 42.1% identity. Unlike the other known PPases, the expression of PP7 is not ubiquitous; PP7 was only detected in retina and retinal-derived Y-79 retinoblastoma cells. Expression of recombinant human PP7 in baculovirus-infected SF21 insect cells produces an active soluble enzyme that is capable of utilizing phosphohistone and p-nitrophenyl phosphate as substrates. The activity of recombinant PP7 is dependent on Mg2+ and is activated by calcium (IC50 approximately equal to 250 microM). PP7 is not affected by calmodulin and is insensitive to inhibition by okadaic acid, microcystin-LR, calyculin A, and cantharidin.

Molecular characterization of a fourth isoform of the catalytic subunit of protein phosphatase 2A from Arabidopsis thaliana

We have recently reported the existence of multiple isoforms of the catalytic subunit of protein phosphatase 2A (PP2A) in Arabidopsis thaliana and the molecular cloning of cDNAs encoding three of these proteins (PP2A-1, PP2A-2, PP2A-3). The reported cDNA encoding PP2A-3 was truncated at the 5' terminus, lacking a short fragment of the N-terminal coding sequence. We have now isolated a near full-length cDNA encoding the entire PP2A-3 protein (313 residues). The clone includes 188 nucleotides of 5'-untranslated region, where a 44 bp long poly(GA) track is found. We also describe the cloning of a cDNA encoding a fourth isoform of PP2A (PP2A-4). The polypeptide contains 313 residues being 98% identical to PP2A-3 and only 80% identical to both PP2A-1 and PP2A-2. The mRNA for PP2A-4 is 1.4 kb in length and, although predominantly expressed in roots, it is also found in other organs. It is concluded that in A. thaliana the isoforms of PP2A can be grouped in two extremely conserved subfamilies.

Selective increases in isoform PP1 alpha of type-1 protein phosphatase in ascites hepatoma cells

The amounts of four isoforms of the catalytic subunit of type-1 protein phosphatases, PP1 alpha, PP1 gamma 1, PP1 gamma 2, and PP1 delta, have been determined in extracts of rat ascites hepatomas, AH131A, AH13, AH13NMOR, AH143A, and Yoshida sarcoma, and compared to those of rat liver by Western blot analysis. The amount of PP1 alpha was increased over three times in all five hepatomas. The amount of PP1 gamma 1 was increased over two times in AH13, AH13NMOR, and AH143A. The amount of PP1 delta was selectively increased about 4 times in AH131A and AH143A. The PP1 gamma 2 protein was undetectable in both liver and hepatomas. There was good parallelism between the general increase in only PP1 alpha protein in the hepatomas and the previous data demonstrating the general increase in PP1 alpha mRNA in numerous ascites hepatomas. These results suggest that PP1 alpha plays important roles in the expression of malignant phenotype, that its amount is under strict control at the transcription level, and that PP1 gamma 1 and PP1 delta play different roles in the expression of some phenotype(s) of the ascites hepatomas.

Protein phosphatases in higher plants: multiplicity of type 2A phosphatases in Arabidopsis thaliana

Two DNA fragments, AP-1 and AP-2, encoding amino acid sequences closely related to Ser/Thr protein phosphatases were amplified from Arabidopsis thaliana genomic DNA. Fragment AP-1 was used to screen A. thaliana cDNA libraries and several positive clones were isolated. Clones EP8a and EP14a were sequenced and found to encode almost identical proteins (97% identity). Both proteins are 306 amino acids in length and are very similar (79-80% identity) to the mammalian isotypes of the catalytic subunit of protein phosphatase 2A. Therefore, they have been designated PP2A-1 and PP2A-2. A third cDNA clone, EP7, was isolated and sequenced. The polypeptide encoded (308 amino acids, lacking the initial Met codon) is 80% identical with human phosphatases 2A and was named PP2A-3. The PP2A-3 protein is extremely similar (95% identity) to the predicted protein from a cDNA clone previously found in Brassica napus. Southern blot analysis of genomic DNA using AP-1 and AP-2 probes, as well as probes derived from clones EP7, EP8a and EP14a strongly indicates that at least 6 genes closely related to type 2A phosphatases are present in the genome of A. thaliana. Northern blot analysis using the same set of probes demonstrates that, at the seedling stage, the mRNA levels for PP2A-1, PP2A-3 and the gene containing the AP-1 sequence are much higher than those of PP2A-2 and AP-2. These results demonstrate that a multiplicity of type 2A phosphatases might be differentially expressed in higher plants.

Expression of multiple type 1 phosphoprotein phosphatases in Arabidopsis thaliana

Type 1 phosphoprotein Ser/Thr phosphatases (PP1) are highly conserved enzymes found in all eukaryotes. These enzymes have multiple functions in fungal and animal cells but little is known of their function and regulation in plants. Previous studies in our laboratory indicated that maize and Arabidopsis contain a family of PP1 genes and/or pseudogenes. In this study, we report the isolation of five distinct Arabidopsis cDNA clones (TOPP1, TOPP2, TOPP3, TOPP4 and TOPP5) which encode the catalytic subunit (PP1c) of type 1 protein phosphatases. Genomic Southern blot analyses indicate that these clones are the products of five distinct genes and that an additional 2-3 PP1c genes and/or pseudogenes may be present in the Arabidopsis genome. The derived amino acid sequences of the TOPP clones are very similar to published sequences of PP1c from animals, fungi and plants. Four of the TOPP amino acid sequences show unique structural features not observed in other PP1c sequences from fungi or animals. All of the TOPP genes are expressed in Arabidopsis roots, rosettes and flowers, although TOPP1, TOPP2 and TOPP3 appear to be expressed at higher levels in these tissues than TOPP4 and TOPP5.

Molecular and phylogenetic analysis of calmodulin-dependent protein phosphatase (calcineurin) catalytic subunit genes

In the mammalian brain, there are multiple catalytic subunits for the Ca(2+)- and calmodulin-dependent protein phosphatase [also called protein phosphatase 2B (PP-2B) and calcineurin] that are derived from two structural genes. The coding sequences of these two genes are distinguished by the absence (PP2B alpha 1) or the presence (PP2B alpha 2) of an amino terminus containing polyproline. Both of these genes can produce intragenic isoforms through alternative splicing. In the present study, a potential phylogenetic relationship of these genes was inferred from analysis of genomic DNA and from studies of mRNA and protein expression. Southern blot analysis showed unique restriction fragments for both genes in seven mammalian species; however, in organisms from two nonmammalian vertebrates (chicken and lizard), hybridization was observed only for PP2B alpha 1. In agreement with these results, Northern blots of mammalian brain RNA showed transcripts for both genes, with about two to three times more of the PP2B alpha 1 mRNAs, whereas in chicken and lizard, only PP2B alpha 1 transcripts were detected. An analysis of protein expression by two-dimensional electrophoresis was also consistent with these findings. For the purified mammalian brain protein, eight to ten variants were observed with isoelectric points of 5.2-5.8; immunoblot analysis using anti-peptide antibodies indicated that the majority of these were PP2B alpha 1 forms. In chicken brain, multiple isoforms were recognized by antibodies against the PP2B alpha 1 forms, but no reactivity was seen with those against the PP2B alpha 2 forms. Taken together, these findings suggest that: (i) in mammals, the predominant catalytic subunit isoforms in brain are PP2B alpha 1 products and (ii) the gene for the polyproline-containing catalytic subunit of calmodulin-dependent phosphatase (PP2B alpha 2) may have evolved after the avian/reptilian branching point, perhaps to carry out a role(s) of particular significance in mammals.

The role of protein phosphatases in synaptic transmission, plasticity and neuronal development

In the past year significant advances have been made in our understanding of the role of protein dephosphorylation in the control of neuronal function. Molecular cloning has identified a large number of serine/threonine and tyrosine protein phosphatases in the nervous system. Many of these enzymes are selectively enriched in the nervous system, some are localized to specific neurons, and yet others are expressed only during specific periods of neuronal development. The availability of purified protein phosphatases and selective inhibitors has facilitated the analysis of these enzymes and their role in the regulation of neurotransmitter receptors and ion channels.