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

Multiple isoforms for the catalytic subunit of PKA in the basal fungal lineage Mucor circinelloides

Protein kinase A (PKA) activity is involved in dimorphism of the basal fungal lineage Mucor. From the recently sequenced genome of Mucor circinelloides we could predict ten catalytic subunits of PKA. From sequence alignment and structural prediction we conclude that the catalytic core of the isoforms is conserved, and the difference between them resides in their amino termini. This high number of isoforms is maintained in the subdivision Mucoromycotina. Each paralogue, when compared to the ones form other fungi is more homologous to one of its orthologs than to its paralogs. All of these fungal isoforms cannot be included in the class I or II in which fungal protein kinases have been classified. mRNA levels for each isoform were measured during aerobic and anaerobic growth. The expression of each isoform is differential and associated to a particular growth stage. We reanalyzed the sequence of PKAC (GI 20218944), the only cloned sequence available until now for a catalytic subunit of M. circinelloides. PKAC cannot be classified as a PKA because of its difference in the conserved C-tail; it shares with PKB a conserved C2 domain in the N-terminus. No catalytic activity could be measured for this protein nor predicted bioinformatically. It can thus be classified as a pseudokinase. Its importance can not be underestimated since it is expressed at the mRNA level in different stages of growth, and its deletion is lethal.

Calcineurin plays key roles in the dimorphic transition and virulence of the human pathogenic zygomycete Mucor circinelloides

Many pathogenic fungi are dimorphic and switch between yeast and filamentous states. This switch alters host-microbe interactions and is critical for pathogenicity. However, in zygomycetes, whether dimorphism contributes to virulence is a central unanswered question. The pathogenic zygomycete Mucor circinelloides exhibits hyphal growth in aerobic conditions but switches to multi-budded yeast growth under anaerobic/high CO₂ conditions. We found that in the presence of the calcineurin inhibitor FK506, Mucor exhibits exclusively multi-budded yeast growth. We also found that M. circinelloides encodes three calcineurin catalytic A subunits (CnaA, CnaB, and CnaC) and one calcineurin regulatory B subunit (CnbR). Mutations in the latch region of CnbR and in the FKBP12-FK506 binding domain of CnaA result in hyphal growth of Mucor in the presence of FK506. Disruption of the cnbR gene encoding the sole calcineurin B subunit necessary for calcineurin activity yielded mutants locked in permanent yeast phase growth. These findings reveal that the calcineurin pathway plays key roles in the dimorphic transition from yeast to hyphae. The cnbR yeast-locked mutants are less virulent than the wild-type strain in a heterologous host system, providing evidence that hyphae or the yeast-hyphal transition are linked to virulence. Protein kinase A activity (PKA) is elevated during yeast growth under anaerobic conditions, in the presence of FK506, or in the yeast-locked cnbR mutants, suggesting a novel connection between PKA and calcineurin. cnaA mutants lacking the CnaA catalytic subunit are hypersensitive to calcineurin inhibitors, display a hyphal polarity defect, and produce a mixture of yeast and hyphae in aerobic culture. The cnaA mutants also produce spores that are larger than wild-type, and spore size is correlated with virulence potential. Our results demonstrate that the calcineurin pathway orchestrates the yeast-hyphal and spore size dimorphic transitions that contribute to virulence of this common zygomycete fungal pathogen.

Protein kinase A is involved in the control of morphology and branching during aerobic growth of Mucor circinelloides

The cAMP signal transduction pathway controls many processes in fungi. The Mucor circinelloides pkaR and pkaC genes, encoding the regulatory (PKAR) and catalytic (PKAC) subunit of the cAMP-dependent protein kinase A (PKA), have been cloned recently. Expression analysis during the dimorphic shift and colony morphology suggested a role for PKAR in the control of morphology and branching. Here strain KFA121, which overexpresses the M. circinelloides pkaR gene, was used to quantify growth and branching under different aerobic growth conditions in a flow-through cell by computerized image analysis. An inverse relationship between the pkaR expression level in KFA121 and the hyphal growth unit length was observed in KFA121, suggesting a central role for PKAR in branching. A biochemical analysis of PKAR using antibodies and enzyme assay demonstrated that the level of PKAR is higher in KFA121 under inducing conditions, i.e. in the presence of high glucose, than in the vector control strain KFA89. Measurement of cAMP binding demonstrated a significant increase (two- to threefold) in PKAR level for KFA121 at the time of germ-tube emission in medium containing 10 g glucose l(-1). The level of PKA activity was determined using kemptide in the same crude cell extracts used to determine cAMP binding. Strain KFA121 showed a twofold increase in PKA activity. An excess of free PKAR subunit over PKA holoenzyme was determined using sucrose gradient centrifugation of extracts from KFA89 and KFA121. The data indicate that cAMP-dependent PKA in M. circinelloides might be down-regulated during hyphal-tube emergence and that an increase in PKAR levels results in increased branching.

Identification and analysis of genes involved in the control of dimorphism in Mucor circinelloides (syn. racemosus)

Mucor circinelloides (syn. racemosus) is a non-pathogenic dimorphic fungus belonging to the class of zygomycetes. We are developing a novel system for heterologous protein production exploiting the dimorphic growth characteristics of M. circinelloides. In order to identify potential genetic regulators of morphology we have initiated a characterisation of key genes involved in signal transduction in Mucor. We have cloned and characterised pkaR and pkaC encoding the regulatory subunit (PKAR) and the catalytic subunit (PKAC), respectively, of the cAMP-dependent protein kinase A (PKA) of M. circinelloides. In anaerobically grown yeast cells, the levels of expression of both pkaR and pkaC were significantly higher than the levels of expression in aerobically grown mycelium. However, during the dimorphic shift, i.e. during the transition from anaerobic yeast growth to aerobic filamentous growth, the expression of pkaR was found to increase approximately two-fold. These results indicate that regulation of PKA activity is conferred at different levels according to growth and environmental conditions. Overexpression of pkaR resulted in a multi-branched colony phenotype on solid medium indicating that PKAR plays a role in filamentation and branching. Fragments of genes encoding factors of the mitogen-activated protein (MAP) kinase (MAPK) pathway have also been cloned: mpk1 (mitogen-activated protein kinase 1) encoding a MAPK homologue and ste12 encoding a transcription factor.

Metabolic engineering of the morphology of Aspergillus

The morphology of filamentous organisms in submerged cultivation is a subject of considerable interest, notably due to the influence of morphology on process productivity. The relationship between process parameters and morphology is complex: the interactions between process variables, productivity, rheology, and macro- and micro-morphology create difficulties in defining and separating cause and effect. Additionally, organism physiology contributes a further level of complexity which means that the desired morphology (for optimum process performance and productivity) is likely to be process specific. However, a number of studies with increasingly powerful image analysis systems have yielded valuable information on what these desirable morphologies are likely to be. In parallel, studies on a variety of morphological mutants means that information on the genes involved in morphology is beginning to emerge. Indeed, we are now beginning to understand how morphology may be controlled at the molecular level. Coupling this knowledge with the tools of molecular biology means that it is now possible to design and engineer the morphology of organisms for specific bioprocesses. Tailor making strains with defined morphologies represents a clear advantage in optimization of submerged bioprocesses with filamentous organisms.

Protein kinase A activity in permeabilized cells as an approximation to in vivo activity

Permeabilized germlings from the dimorphic fungus Mucor rouxii were used for in situ measurement of protein kinase A (PKA) activation, to compare the results with those obtained in vitro at low or high (nonlinear) enzyme concentrations. The apparent total activity per cell when measured in situ is 5- to 10-fold lower than the in vitro measured activity in crude extracts from those cells. Polyamines and NaCl stimulate the activity in situ. The apparent relative specific activity of the in situ measured PKA toward four peptide substrates is similar to the results obtained in vitro at high holoenzyme concentration and not to those obtained with the free catalytic subunit. Saturation in the activation of PKA by cAMP in situ is attained at low concentrations (2 to 10 microM), while in vitro, at high holoenzyme concentration, no saturation was attained up to 1 mM cAMP (V. Zaremberg et al. Arch. Biochem. Biophys. 381, 74-82, 2000). Activation of PKA by site-selective cAMP analogs is assayed in situ and in vitro at two enzyme concentrations. Site B-selective cAMP analogs are good activators of PKA at low enzyme concentration in vitro but poor activators either at high enzyme concentration in vitro or in permeabilized cells. A physiological correlation with the behavior of site-selective analogs in situ is demonstrated in vivo when assaying the effect of increasing concentrations of site-selective cAMP analogs on the impairment of polarized growth of M. rouxii spores.

Threshold level of protein kinase A activity and polarized growth in Mucor rouxii

A model system to study the involvement of cAMP-mediated regulation of a cellular process such as hyphal morphogenesis was investigated. Impairment of polarized growth was observed when Mucor rouxii sporangiospores were grown in the presence of N(6)-cAMP analogues and of SQ 65,442, a cAMP phosphodiesterase inhibitor. Together with an effect on isodiametric growth, there was increased pigmentation, increased cell fragility and loss of cell adhesiveness. The total effect on morphology was attained even after adding the compounds shortly before germ-tube emergence; when added after this time growth continued in a non-polarized form and rounding of the germ tip was observed. The morphological effect was observed under all the nutritional and environmental conditions studied (aerobic conditions and defined medium with maltose or glucose, Casamino acids medium with glucose, or rich medium; anaerobic conditions with rich medium; and following a shift from anaerobiosis to aerobiosis). The time of germ-tube emergence, and the size of the cell at this time, was dependent on the growth medium. Protein kinase A (PKA) specific activity was followed during the germination process under three growth conditions. It was found that the total activity of PKA correlated with differentiation and not with growth, and that the total specific activity at the time of germination was the same, independent of the culture medium. The time of germ-tube emergence correlated with the time of attainment of a threshold level of PKA total specific activity. The concentration of dibutyryl-cAMP needed to promote isodiametric growth correlated with the total units of activity of PKA to be activated per cell. It was concluded that PKA is involved in the morphogenetic process of the fungus grown under all the nutritional and ambient conditions tested.

Characterization and overexpression of the Aspergillus niger gene encoding the cAMP-dependent protein kinase catalytic subunit

The gene pkaC encoding the catalytic subunit of cAMP-dependent protein kinase has been isolated from the industrially important filamentous fungus Aspergillus niger. A probe for screening A. niger phage libraries was generated by a polymerase chain reaction using degenerate primers. cDNA and genomic DNA clones were isolated and sequenced. An open reading frame of 1440 bp, interrupted by three short introns, encodes a polypeptide of 480 amino acids with a calculated molecular mass of 53813 Da. The cAMP-dependent protein kinase catalytic subunit (PKA-C) from A. niger has a 126 amino acid extension at the N-terminus compared to the PKA-C of higher eukaryotes that-except for the first 15 amino acids, which are homologous to the Magnaporthe grisea PKA-C-shows no significant similarity to the N-terminal extension of PKA-C of other lower eukaryotes. The catalytic core of PKA-C of A. niger shows extensive homology with the PKA-C isolated from all other eukaryotes. Low-stringency hybridization did not reveal any other pkaC homologue in A. niger. The cloned pkaC was used for transformation of A. niger, leading to increased levels of pkaC mRNA and PKA-C activity. Transformants overexpressing pkaC were phenotypically different with respect to growth, showing a more compact colony morphology, accompanied by a more dense sporulation, especially on media containing trehalose and glycerol. A number of transformants also showed a strongly reduced or complete absence of sporulation. This phenotype was quickly lost upon propagation of the strains.