Phenotypic analysis of human glioma cells expressing the MMAC1 tumor suppressor phosphatase.
MMAC1, also known as PTEN or TEP-1, was recently identified as a gene commonly mutated in a variety of human neoplasias. Sequence analysis revealed that MMAC1 harbored sequences similar to those found in several protein phosphatases. Subsequent studies demonstrated that MMAC1 possessed in vitro enzymatic activity similar to that exhibited by dual specificity phosphatases. To characterize the potential cellular functions of MMAC1, we expressed wild-type and several mutant variants of MMAC1 in the human glioma cell line, U373, that lacks endogenous expression. While expression of wild-type MMAC1 in these cells significantly reduced their growth rate and saturation density, expression of enzymatically inactive MMAC1 significantly enhanced growth in soft agar. Our observations indicate that while wild-type MMAC1 exhibits activities compatible with its proposed role as a tumor suppressor, cellular expression of MMAC1 containing mutations in the catalytic domain may yield protein products that enhance transformation characteristics. (+info)
A processive single-headed motor: kinesin superfamily protein KIF1A.
A single kinesin molecule can move "processively" along a microtubule for more than 1 micrometer before detaching from it. The prevailing explanation for this processive movement is the "walking model," which envisions that each of two motor domains (heads) of the kinesin molecule binds coordinately to the microtubule. This implies that each kinesin molecule must have two heads to "walk" and that a single-headed kinesin could not move processively. Here, a motor-domain construct of KIF1A, a single-headed kinesin superfamily protein, was shown to move processively along the microtubule for more than 1 micrometer. The movement along the microtubules was stochastic and fitted a biased Brownian-movement model. (+info)
Characterization of the interaction domains of Ure2p, a prion-like protein of yeast.
In the yeast Saccharomyces cerevisiae, the non-Mendelian inherited genetic element [URE3] behaves as a prion. A hypothesis has been put forward which states that [URE3] arises spontaneously from its cellular isoform Ure2p (the product of the URE2 gene), and propagates through interactions of the N-terminal domain of the protein, thus leading to its aggregation and loss of function. In the present study, various N- and C-terminal deletion mutants of Ure2p were constructed and their cross-interactions were tested in vitro and in vivo using affinity binding and a two-hybrid analysis. We show that the self-interaction of the protein is mediated by at least two domains, corresponding to the first third of the protein (the so-called prion-forming domain) and the C-terminal catalytic domain. (+info)
Purification and identification of a novel subunit of protein serine/threonine phosphatase 4.
The catalytic subunit of protein serine/threonine phosphatase 4 (PP4C) has greater than 65% amino acid identity to the catalytic subunit of protein phosphatase 2A (PP2AC). Despite this high homology, PP4 does not appear to associate with known PP2A regulatory subunits. As a first step toward characterization of PP4 holoenzymes and identification of putative PP4 regulatory subunits, PP4 was purified from bovine testis soluble extracts. PP4 existed in two complexes of approximately 270-300 and 400-450 kDa as determined by gel filtration chromatography. The smaller PP4 complex was purified by sequential phenyl-Sepharose, Source 15Q, DEAE2, and Superdex 200 gel filtration chromatographies. The final product contained two major proteins: the PP4 catalytic subunit plus a protein that migrated as a doublet of 120-125 kDa on SDS-polyacrylamide gel electrophoresis. The associated protein, termed PP4R1, and PP4C also bound to microcystin-Sepharose. Mass spectrometry analysis of the purified complex revealed two major peaks, at 35 (PP4C) and 105 kDa (PP4R1). Amino acid sequence information of several peptides derived from the 105 kDa protein was utilized to isolate a human cDNA clone. Analysis of the predicted amino acid sequence revealed 13 nonidentical repeats similar to repeats found in the A subunit of PP2A (PP2AA). The PP4R1 cDNA clone engineered with an N-terminal Myc tag was expressed in COS M6 cells and PP4C co-immunoprecipitated with Myc-tagged PP4R1. These data indicate that one form of PP4 is similar to the core complex of PP2A in that it consists of a catalytic subunit and a "PP2AA-like" structural subunit. (+info)
PrKX is a novel catalytic subunit of the cAMP-dependent protein kinase regulated by the regulatory subunit type I.
The human X chromosome-encoded protein kinase X (PrKX) belongs to the family of cAMP-dependent protein kinases. The catalytically active recombinant enzyme expressed in COS cells phosphorylates the heptapeptide Kemptide (LRRASLG) with a specific activity of 1.5 micromol/(min.mg). Using surface plasmon resonance, high affinity interactions were demonstrated with the regulatory subunit type I (RIalpha) of cAMP-dependent protein kinase (KD = 10 nM) and the heat-stable protein kinase inhibitor (KD = 15 nM), but not with the type II regulatory subunit (RIIalpha, KD = 2.3 microM) under physiological conditions. Kemptide and autophosphorylation activities of PrKX are strongly inhibited by the RIalpha subunit and by protein kinase inhibitor in vitro, but only weakly by the RIIalpha subunit. The inhibition by the RIalpha subunit is reversed by addition of nanomolar concentrations of cAMP (Ka = 40 nM), thus demonstrating that PrKX is a novel, type I cAMP-dependent protein kinase that is activated at lower cAMP concentrations than the holoenzyme with the Calpha subunit of cAMP-dependent protein kinase. Microinjection data clearly indicate that the type I R subunit but not type II binds to PrKX in vivo, preventing the translocation of PrKX to the nucleus in the absence of cAMP. The RIIalpha subunit is an excellent substrate for PrKX and is phosphorylated in vitro in a cAMP-independent manner. We discuss how PrKX can modulate the cAMP-mediated signal transduction pathway by preferential binding to the RIalpha subunit and by phosphorylating the RIIalpha subunit in the absence of cAMP. (+info)
Mechanistic studies on the reductive half-reaction of NADPH-cytochrome P450 oxidoreductase.
Site-directed mutagenesis has been employed to study the mechanism of hydride transfer from NADPH to NADPH-cytochrome P450 oxidoreductase. Specifically, Ser457, Asp675, and Cys630 have been selected because of their proximity to the isoalloxazine ring of FAD. Substitution of Asp675 with asparagine or valine decreased cytochrome c reductase activities 17- and 677-fold, respectively, while the C630A substitution decreased enzymatic activity 49-fold. Earlier studies had shown that the S457A mutation decreased cytochrome c reductase activity 90-fold and also lowered the redox potential of the FAD semiquinone (Shen, A., and Kasper, C. B. (1996) Biochemistry 35, 9451-9459). The S457A/D675N and S457A/D675N/C630A mutants produced roughly multiplicative decreases in cytochrome c reductase activity (774- and 22000-fold, respectively) with corresponding decreases in the rates of flavin reduction. For each mutation, increases were observed in the magnitudes of the primary deuterium isotope effects with NADPD, consistent with decreased rates of hydride transfer from NADPH to FAD and an increase in the relative rate limitation of hydride transfer. Asp675 substitutions lowered the redox potential of the FAD semiquinone. In addition, the C630A substitution shifted the pKa of an ionizable group previously identified as necessary for catalysis (Sem, D. S., and Kasper, C. B. (1993) Biochemistry 32, 11539-11547) from 6.9 to 7.8. These results are consistent with a model in which Ser457, Asp675, and Cys630 stabilize the transition state for hydride transfer. Ser457 and Asp675 interact to stabilize both the transition state and the FAD semiquinone, while Cys630 interacts with the nicotinamide ring and the fully reduced FAD, functioning as a proton donor/acceptor to FAD. (+info)
Characterization of transgenic mice with targeted disruption of the catalytic domain of the double-stranded RNA-dependent protein kinase, PKR.
The interferon-inducible, double-stranded RNA-dependent protein kinase PKR has been implicated in anti-viral, anti-tumor, and apoptotic responses. Others have attempted to examine the requirement of PKR in these roles by targeted disruption at the amino terminal-encoding region of the Pkr gene. By using a strategy that aims at disruption of the catalytic domain of PKR, we have generated mice that are genetically ablated for functional PKR. Similar to the other mouse model of Pkr disruption, we have observed no consequences of loss of PKR on tumor suppression. Anti-viral response to influenza and vaccinia also appeared to be normal in mice and in cells lacking PKR. Cytokine signaling in the type I interferon pathway is normal but may be compromised in the erythropoietin pathway in erythroid bone marrow precursors. Contrary to the amino-terminal targeted Pkr mouse, tumor necrosis factor alpha-induced apoptosis and the anti-viral apoptosis response to influenza is not impaired in catalytic domain-targeted Pkr-null cells. The observation of intact eukaryotic initiation factor-2alpha phosphorylation in these Pkr-null cells provides proof of rescue by another eukaryotic initiation factor-2alpha kinase(s). (+info)
His ... Asp catalytic dyad of ribonuclease A: histidine pKa values in the wild-type, D121N, and D121A enzymes.
Bovine pancreatic ribonuclease A (RNase A) has a conserved His ... Asp catalytic dyad in its active site. Structural analyses had indicated that Asp121 forms a hydrogen bond with His119, which serves as an acid during catalysis of RNA cleavage. The enzyme contains three other histidine residues including His12, which is also in the active site. Here, 1H-NMR spectra of wild-type RNase A and the D121N and D121A variants were analyzed thoroughly as a function of pH. The effect of replacing Asp121 on the microscopic pKa values of the histidine residues is modest: none change by more than 0.2 units. There is no evidence for the formation of a low-barrier hydrogen bond between His119 and either an aspartate or an asparagine residue at position 121. In the presence of the reaction product, uridine 3'-phosphate (3'-UMP), protonation of one active-site histidine residue favors protonation of the other. This finding is consistent with the phosphoryl group of 3'-UMP interacting more strongly with the two active-site histidine residues when both are protonated. Comparison of the titration curves of the unliganded enzyme with that obtained in the presence of different concentrations of 3'-UMP shows that a second molecule of 3'-UMP can bind to the enzyme. Together, the data indicate that the aspartate residue in the His ... Asp catalytic dyad of RNase A has a measurable but modest effect on the ionization of the adjacent histidine residue. (+info)