Protein kinase from Mucor rouxii. Unshielding of new cyclic AMP binding sites upon dissociation of the ternary complex holoenzyme-cyclic AMP

A novel activating effect of the regulatory subunit of protein kinase A on catalytic subunit activity

The strength of the interaction between the catalytic and regulatory subunits in protein kinase A differs among species. The linker region from regulatory subunits is non-conserved. To evaluate the participation of this region in the interaction with the catalytic subunit, we have assayed its effect on the enzymatic properties of the catalytic subunit. Protein kinase A from three fungi, Mucor rouxii, Mucor circinelloides and Saccharomyces cerevisiae have been chosen as models. The R-C interaction is explored by using synthetic peptides of 8, 18 and 47 amino acids, corresponding to the R subunit autophosphorylation site plus a variable region toward the N terminus (0, 10, or 39 residues). The K(m) of the catalytic subunits decreased with the length of the peptide, while the V(max) increased. Viscosity studies identified product release as the rate limiting step for phosphorylation of the longer peptides. Pseudosubstrate derivatives of the 18 residue peptides did not display a competitive inhibition behavior toward either kemptide or a bona fide protein substrate since, at low relative pseudosubstrate/substrate concentration, stimulation of kemptide or protein substrate phosphorylation was observed. The behavior was mimicked by intact R. We conclude that in addition to its negative regulatory role, the R subunit stimulates C activity via distal interactions.

Mechanism of activation of cAMP-dependent protein kinase: in Mucor rouxii the apparent specific activity of the cAMP-activated holoenzyme is different than that of its free catalytic subunit

Kinetic constants for peptide phosphorylation by the catalytic subunit of the dimorphic fungus Mucor rouxii protein kinase A were determined using 13 peptides derived from the peptide containing the basic consensus sequence RRASVA, plus kemptide, S6 peptide, and protamine. As a whole, although with a greater Km, the order of preference of the peptides by the M. rouxii catalytic subunit was similar to the one displayed by mammalian protein kinase A. Particularly significant is the replacement of serine by threonine in the basic peptide RRATVA, which impaired its role as a substrate of M. rouxii catalytic subunit. Mucor rouxii protein kinase A is a good model in which to study the mechanism of activation since cAMP alone is not enough to promote activation and dissociation. Four peptides were selected for the study of holoenzyme activation under conditions in which the enzymatic activity was not proportional to the holoenzyme concentration: RRASVA, RRRRASVA, KRRRLSSRA (S6 peptide), and LRRASLG (kemptide); protamine was used as reference. Differential activation degree was observed depending on the peptide used and on cAMP concentration. Ratios of activity between different substrates displayed by the holoenzyme under the above conditions did not reflect the one expected for the free catalytic subunit. The degree of inhibition of the holoenzyme activity by an active peptide derived from the thermostable protein kinase inhibitor was dependent on the substrate used and on the holoenzyme concentration, while it was found to be independent of these two parameters for free catalytic subunit. Polycation modulation of holoenzyme activation by cAMP was also dependent on the polycation itself and on the peptide used as substrate. The observed kinetic differences between holoenzyme and free catalytic subunit were decreased or almost abolished when working at low enzyme or at high cAMP concentrations. Two hypotheses compatible with the results are discussed: substrate participation in the dissociation process and/or holoenzyme activation without dissociation.

Analysis of the mechanism of activation of cAMP-dependent protein kinase through the study of mutants of the yeast regulatory subunit

Spontaneous mutations in the gene which encodes the regulatory subunit of cAMP-dependent protein kinase (PKA) of Saccharomyces cerevisiae (BCY1) have been isolated previously [Cannon, J. F., Gibbs, J. B. & Tatchell, K. (1986) Genetics 113, 247-264] by selection of ras2::LEU2 revertants that grew on non-fermentable carbon sources. The revertants were placed into groups of increasing severity based on the number of PKA-dependent traits affected [Cannon, J. F., Gitan, R. & Tatchell, K. (1990) J. Biol. Chem. 265, 11897-11904]. In this work the ras2 mutation has been crossed out in each bcy1 allele and the phenotypes of these mutants have been assessed. The order of severity of the mutants in both genetic backgrounds is maintained but the severity of each mutant in the normal background is higher than in the ras2::LEU2 background. Total catalytic-subunit and regulatory-subunit activities were measured in crude extracts of the bcy1 ras2::LEU2 mutants. With one exception (bcy1-6) the calculated regulatory subunit/catalytic subunit ratios of the bcy1 mutants relative to that of wild-type cells were greater than one. The dependence of PKA activity on cAMP was measured in permeabilized cells. The strains show an activity ratio in the absence and presence of cAMP in the range 0.5-1 for Kemptide phosphorylation. Overexpression of the high-affinity cAMP phosphodiesterase gene (PDE2) in the bcy1 ras2::LEU2 strains did not alter their PKA-dependent phenotypes. However, transformants were not observed from the parental ras2::LEU2 strain and the bcy1-6 ras2::LEU2 strain. The results are discussed with respect to a hypothesis for the molecular mechanism of the differential reversal of ras2 phenotypes by the bcy1 alleles. Mutations in the regulatory subunit are predicted to affect the structure of the holoenzyme such that the catalytic subunit is capable of maintaining an active catalytic state, without the need to dissociate from the regulatory subunit.

Regulation of protein kinase A subunits during germination of Mucor rouxii sporangiospores

Levels of protein kinase A (PKA) subunits and of cAMP have been measured during aerobic germination of the sporangiospores of the dimorphic fungus Mucor rouxii; further, the holoenzyme and its catalytic (C) and regulatory (R) subunits have been visualized through sucrose gradient centrifugation. Sporangiospores contain around 0.06 microM of a dimeric holoenzyme species of 5.5 S and a sixfold excess of a free R subunit of 2.7 S. Both these species are proposed to be derived by proteolysis from the native forms. Enzymic activity at this stage is highly inhibited, as demonstrated with permeabilized cells. Immediately upon germination, and after a transient increase in cAMP concentration from 10 microM to 90 microM, C-subunit levels fall to 30%. After the onset of germination, the specific activity and concentration of both the 5.5 S holoenzyme species and the 2.7 S species of free R subunit decrease in parallel to the increase in total protein and volume. Net synthesis of C and R subunits to form a native holoenzyme species of 8.8 S is apparent 4 h onwards after germination. A significant increase in cellular concentration is observed at 6 h. At 7 h of growth, when germ-tube emission is complete, the holoenzyme concentration is around 0.23 microM; there is almost no free R subunit and the intracellular concentration of cAMP is around 3 microM. A role for PKA during germination and morphogenesis is discussed.

Isobutylmethylxanthine and other classical cyclic nucleotide phosphodiesterase inhibitors affect cAMP-dependent protein kinase activity

The effect of 17 inhibitors of cyclic nucleotide phosphodiesterases (PDEs) was assayed on cAMP binding activity of Mucor rouxii protein kinase A (PKA), on PKA activity in the absence of cAMP and on free catalytic subunit (C) activity. Isobutylmethylxanthine (IBMX), SQ 20,009 and cilostamide, at 0.2 mM, behaved as partial agonists of cAMP since they inhibited binding of 0.15 microM [3H]cAMP to the regulatory subunit (R), stimulated slightly PKA activity in the absence of cAMP and did not modify C activity. Amrinone at 0.2 mM inhibited C activity competitively towards ATP. These four compounds displayed the same effects when assayed on eukaryotic protein kinase A types I (PKI) and II (PKII). The combined effect of IBMX and cAMP was analysed on Mucor PKA. Under dissociating conditions (+ 0.5 M NaCl) IBMX antagonized activation by low concentrations of cAMP, while in the absence of NaCl, IBMX potentiated the stimulating activity of cAMP.

Autophosphorylation of Mucor rouxii cAMP-dependent protein kinase and its role in holoenzyme activation

Two phosphoproteins of 53,000 and 63,000 mol. wt detected in partially purified preparations of Mucor rouxii cAMP-dependent protein kinase submitted to phosphorylation conditions with [gamma-32P]ATP are demonstrated to be the result of the autophosphorylation of its regulatory subunit, according to the following criteria: (1) linearity of phosphate incorporation with enzyme sample; (2) independence of phosphate incorporation on temperature; (3) correlation of the phosphoproteins with enzymatic activity in a DEAE-Sepharose chromatography; (4) specific elution of the phosphorylated proteins from cAMP-agarose; (5) phosphorylation of the purified regulatory subunit. Antibodies specific against Mucor regulatory subunit detected an intact subunit of 72,000 mol. wt in crude extracts. Autophosphorylation of the fungal protein kinase A promotes activation of the holoenzyme by cAMP since: (1) under conditions of partial activation, increase of activity is observed when using the phosphoform of the enzyme; (2) release of free catalytic subunit from cAMP-agarose is enhanced when the holoenzyme is previously phosphorylated.

Mucor dimorphism

Mucor dimorphism has interested microbiologists since the time of Pasteur. When deprived of oxygen, these fungi grow as spherical, multipolar budding yeasts. In the presence of oxygen, they propagate as branching coenocytic hyphae. The ease with which these morphologies can be manipulated in the laboratory, the diverse array of morphopoietic agents available, and the alternative developmental fates that can be elicited from a single cell type (the sporangiospore) make Mucor spp. a highly propitious system in which to study eukaryotic cellular morphogenesis. The composition and organization of the cell wall differ greatly in Mucor yeasts and hyphae. The deposition of new wall polymers is isodiametric in yeasts and apically polarized in hyphae. Current research has focused on the identity and control of enzymes participating in wall synthesis. An understanding of how the chitosome interacts with appropriate effectors, specific enzymes, and the plasma membrane to assemble chitin-chitosan microfibrils and to deposit them at the proper sites on the cell exterior will be critical to elucidating dimorphism. Several biochemical and physiological parameters have been reported to fluctuate in a manner that correlates with Mucor morphogenesis. The literature describing these has been reviewed critically with the intent of distinguishing between causal and casual connections. The advancement of molecular genetics has afforded powerful new tools that researchers have begun to exploit in the study of Mucor dimorphism. Several genes, some encoding products known to correlate with development in Mucor spp. or other fungi, have been cloned, sequenced, and examined for transcriptional activity during morphogenesis. Most have appeared in multiple copies displaying independent transcriptional control. Selective translation of stored mRNA molecules occurs during sporangiospore germination. Many other correlates of Mucor morphogenesis, presently described but not yet explained, should prove amenable to analysis by the emerging molecular technology.

Polyamines and basic proteins stimulate activation by cAMP and catalytic activity of Mucor rouxii cAMP-dependent protein kinase

Partial activation of Mucor rouxii cAMP-dependent protein kinase by cAMP was obtained when kemptide was used as substrate, but complete activation was attained with cAMP plus protamine or histone. Full activation could not be achieved by increasing kemptide or cAMP concentration. Complete activation by cAMP could be obtained by addition of 10 microM polylysine, 10 microM lysine-rich histone or 0.5 mM spermine plus spermidine. The degree of stimulation could be up to 5-fold, depending on the amount of enzyme in the assay. The same concentrations of polycations increased 1.5-2.3-fold the Vmax of kemptide phosphorylation by the free catalytic subunits of both Mucor and bovine heart protein kinases; 10 microM polyarginine inhibited completely the activity of both enzymes.

Two different intrachain cAMP sites in the cAMP-dependent protein kinase of the dimorphic fungus Mucor rouxii

cAMP sites of the cAMP-dependent protein kinase from the fungus Mucor rouxii have been characterized through the study of the effects of cAMP and of cAMP analogs on the phosphotransferase activity and through binding kinetics. The tetrameric holoenzyme, which contains two regulatory (R) and two catalytic (C) subunits, exhibited positive cooperativity in activation by cAMP, suggesting multiple cAMP-binding sites. Several other results indicated that the Mucor kinase contained two different cooperative cAMP-binding sites on each R subunit, with properties similar to those of the mammalian cAMP-dependent protein kinase. Under optimum binding conditions, the [3H]cAMP dissociation behavior indicated equal amounts of two components which had dissociation rate constants of 0.09 min-1 (site 1) and 0.90 min-1 (site 2) at 30 degrees C. Two cAMP-binding sites could also be distinguished by C-8 cAMP analogs (site-1-selective) and C-6 cAMP analogs (site-2-selective); combinations of site-1- and site-2-selective analogs were synergistic in protein kinase activation. The two different cooperative binding sites were probably located on the same R subunit, since the proteolytically derived dimeric form of the enzyme, which contained one R and one C component, retained the salient properties of the untreated tetrameric enzyme. Unlike any of the mammalian cyclic-nucleotide-dependent isozymes described thus far, the Mucor kinase was much more potently activated by C-6 cAMP analogs than by C-8 cAMP analogs. In the ternary complex formed by the native Mucor tetramer and cAMP, only the two sites 1 contained bound cAMP, a feature which has also not yet been demonstrated for the mammalian cAMP-dependent protein kinase.