The regulatory subunit of cAMP-dependent protein kinase from Dictyostelium discoideum: cellular localization and developmental regulation analyzed by immunoblotting

Role of PKA in the timing of developmental events in Dictyostelium cells

The cyclic AMP (cAMP)-dependent protein kinase, PKA, is dispensable for growth of Dictyostelium cells but plays a variety of crucial roles in development. The catalytic subunit of PKA is inhibited when associated with its regulatory subunit but is activated when cAMP binds to the regulatory subunit. Deletion of pkaR or overexpression of the gene encoding the catalytic subunit, pkaC, results in constitutive activity. Development is independent of cAMP in strains carrying these genetic alterations and proceeds rapidly to the formation of both spores and stalk cells. However, morphogenesis is aberrant in these mutants. In the wild type, PKA activity functions in a circuit that can spontaneously generate pulses of cAMP necessary for long-range aggregation. It is also essential for transcriptional activation of both prespore and prestalk genes during the slug stage. During culmination, PKA functions in both prespore and prestalk cells to regulate the relative timing of terminal differentiation. A positive feedback loop results in the rapid release of a signal peptide, SDF-2, when prestalk cells are exposed to low levels of SDF-2. The signal transduction pathway that mediates the response to SDF-2 in both prestalk and prespore cells involves the two-component system of DhkA and RegA. When the cAMP phosphodiesterase RegA is inhibited, cAMP accumulates and activates PKA, leading to vacuolation of stalk cells and encapsulation of spores. These studies indicate that multiple inputs regulate PKA activity to control the relative timing of differentiations in Dictyostelium.

Dictyostelium development in the absence of cAMP

Adenosine 3',5'-monophosphate (cAMP) and cAMP-dependent protein kinase (PKA) are regulators of development in many organisms. Dictyostelium uses cAMP as an extracellular chemoattractant and as an intracellular signal for differentiation. Cells that are mutant in adenylyl cyclase do not develop. Moderate expression of the catalytic subunit of PKA in adenylyl cyclase-null cells led to near-normal development without detectable accumulation of cAMP. These results suggest that all intracellular cAMP signaling is effected through PKA and that signals other than extracellular cAMP coordinate morphogenesis in Dictyostelium.

The cellular content of Cdc25p, the Ras exchange factor in Saccharomyces cerevisiae, is regulated by destabilization through a cyclin destruction box

The Cdc25p and Sdc25p proteins were the first members of the family of guanine nucleotide exchange factors to be identified. These proteins promote the formation of active Ras-GTP complex from inactive Ras-GDP complex by exchange of GDP for GTP. Therefore Cdc25p which is the main positive regulator of Ras, regulates through Ras the activity of adenylate cyclase in Saccharomyces cerevisiae. The amino-terminal part of Cdc25p has a sequence similar to the cyclin destruction box (CDB) of mitotic cyclins. This sequence has been reported to be required for ubiquitin-dependent proteolysis. In this study we show that Cdc25p is an unstable polypeptide with a half-life of 15-20 min. Its instability depends upon the presence of the CDB which can also confer instability to other proteins. Degradation of Cdc25p and CDB containing beta-galactosidase was found to be independent of various cell cycle arrest points. The fast degradation of Cdc25p opens the possibility that Ras and the cAMP cascade in yeast are directly modulated by the cellular content of the guanine nucleotide exchange factor rather than variation in activity or localization control.

Patterns of cyclic AMP-dependent protein kinase gene expression during ontogeny of the murine palate

Normal growth and differentiation of embryonic palatal tissue depends on regulated levels of intracellular cAMP. Cyclic AMP-dependent protein kinases (PKA) act to mediate the biological activities of cAMP. PKA isozyme protein profiles demonstrate a clear pattern of temporal alterations in embryonic palatal tissue during its development. In order to ascertain the molecular basis for changing PKA isozyme profiles during palatal ontogeny, the spatial and temporal expression of mRNAs for regulatory (RI alpha, RII alpha, and RII beta) and catalytic (C alpha) subunits of PKA was examined. RNA extracted from murine embryonic palatal tissue (days 12-14 of gestation) was examined by Northern blot analysis. Significant levels of constitutively expressed RI alpha and C alpha mRNA were seen on all days of gestation examined. RI alpha transcripts were substantially less abundant in palate mesenchymal cells in vitro than in palatal tissue in vivo. Levels of RII alpha and RII beta mRNA were highest on gestational day (GD) 12, a period characterized by pronounced palatal tissue growth. In addition, patterns of tissue distribution of RII beta, not previously described, were examined in the developing embryonic palate. A dramatic developmental shift in tissue distribution of RII beta was seen. The isozyme was evenly distributed between palatal epithelial and mesenchymal cells on GD 12 but by GD 14, RII beta was predominantly localized to palatal epithelial cells. Direct activation of adenylate cyclase with forskolin in murine embryonic palate mesenchymal (MEPM) cells resulted in an increase in RII alpha mRNA levels but had no effect on steady state levels of RII beta or C alpha mRNA. In addition, elevation of intracellular levels of cAMP resulted in a shift in the transcriptional profile of RI alpha mRNAs. Results of this study document specific patterns of expression for the genes encoding the various cAMP-dependent protein kinase regulatory and C alpha subunits in murine embryonic palatal tissue. In addition, we have demonstrated adaptational changes of this kinase in MEPM cells in response to conditions of increased intracellular levels of cAMP.

An unusual catalytic subunit for the cAMP-dependent protein kinase of Dictyostelium discoideum

The cAMP-dependent protein kinase (cAPK) plays an essential role during differentiation and fruit morphogenesis in Dictyostelium discoideum. The presence of an open reading frame on the gene, pkaC (previously named either Dd PK2 or Dd PK3 by different groups), predicts a 73-kDa polypeptide with 54% similarity to the catalytic subunits of cAPKs from other organisms. Using anti-peptide antibodies, we show that the pkaC gene product, PkaC, is a 73-kDa polypeptide. Despite the fact that PkaC is about twice the size of its mammalian counterparts, it possesses all of the properties required of a catalytic subunit. It is physically associated with the regulatory subunit, and this association results in an inhibition of the catalytic activity which is reverted by cAMP. PkaC copurifies with cAPK activity, and an increased cAPK activity is observed in cells overexpressing PkaC. We conclude that PkaC is a catalytic subunit of the Dictyostelium discoideum cAPK and discuss the unusual features of this protein with the highest molecular weight of known cAPKs.

Regulation of Dictyostelium morphogenesis by cAMP-dependent protein kinase

During formation of the Dictyostelium slug extracellular cAMP signals direct the differentiation of prespore cells and DIF, a chlorinated hexaphenone, induces the differentiation of prestalk cells. At culmination the slug transforms into a fruiting body, composed of a stalk supporting a ball of spores. A dominant inhibitor of cAMP-dependent protein kinase (PKA) expressed under the control of a prestalk-specific promoter blocks the differentiation of prestalk cells into stalk cells. Analysis of a gene specifically expressed in stalk cells suggests that PKA acts to remove a repressor that prevents the premature induction of stalk cell differentiation by DIF during slug migration. PKA is also necessary for the morphogenetic movement of prestalk cells at culmination. Expression of the PKA inhibitor under control of a prespore-specific promoter blocks the accumulation of prespore mRNA sequences and prevents terminal spore cell differentiation. Thus PKA is essential for progression along both pathways of terminal differentiation but with different mechanisms of action. On the stalk cell pathway it acts to regulate the action of DIF while on the spore cell pathway PKA itself seems to act as the inducer of spore cell maturation. Ammonia, the extracellular signal which regulates the entry into culmination, acts by controlling the intracellular concentration of cAMP and thus exerts its effects via PKA. The fact that PKA is necessary for both prespore and spore gene expression leads us to postulate the existence of a signalling mechanism which converts the progressive rise in cAMP concentration during development into discrete, PKA-regulated gene activation events.

Isolation of two genes encoding putative protein kinases regulated during Dictyostelium discoideum development

Two Dictyostelium discoideum protein kinase(PK)-encoding cDNAs (Dd PK1 and Dd PK2) have been isolated by hybridization with an oligodeoxyribonucleotide derived from a highly conserved region of eukaryotic PKs. The two nucleotide (nt) sequences encode new putative serine/threonine-specific PKs. Dd PK1 is a partial cDNA covering the entire catalytic domain. The derived amino acid (aa) sequence is about 30% identical to both cAMP-dependent protein kinase (cAPK) and protein kinase C. The Dd PK2 sequence was extended through the isolation of a genomic fragment encoding a complete putative protein. A single intron is present, as deduced from sequence comparison with the cDNA. The catalytic domain appears more closely related to the catalytic subunit of cAPK (54% sequence identity). However, our nt sequence potentially codes for a much larger protein (648 vs. about 350 aa for most cAPKs) with a N-terminal half containing long homopolymers of threonines, glutamines and asparagines. Similar repeats occur at the C terminus of Dd PK1, Dd PK1 is expressed in vegetatively growing cells and during development. Dd PK1 RNA decreases after 6 h of starvation to re-accumulate once the cells have aggregated. Dd PK2 transcripts, present at a low amount in growing cells, rise upon starvation. A switch to a shorter form of transcripts occurs between 3 and 6 h into development.

The Saccharomyces cerevisiae CDC25 gene product is a 180 kDa polypeptide and is associated with a membrane fraction

In the yeast Saccharomyces cerevisiae, the CDC25 gene product is supposed to interact with ras proteins and adenylate cyclase for progression through the cell division cycle. To identify the CDC25 gene product, we raised antibodies against two hybrid proteins, encoded by in-frame fusions between the E. coli lacZ gene and two different parts of the CDC25 gene. By protein immuno-blotting, we were able to identify the CDC25 gene product as a 180 kDa polypeptide, which we named p180CDC25. It was detected only when the CDC25 gene was overexpressed in a proteases-deficient yeast strain. Subcellular fractionation experiments showed that p180CDC25, as well as ras proteins, is attached to the membrane, even after treatments which release peripheral membrane proteins.

Developmental regulation of expression of the regulatory subunit of the cAMP-dependent protein kinase of Blastocladiella emersonii

A monospecific polyclonal antiserum to the regulatory subunit (R) of the cAMP-dependent protein kinase of Blastocladiella emersonii has been developed by immunization with purified regulatory subunit. In Western blots, the antiserum displays high affinity and specificity for the intact R monomer of Mr = 58,000, as well as for its proteolytic products of Mr = 43,000 and Mr = 36,000, even though the antiserum has been raised against the Mr = 43,000 fragment. Western blots of cell extracts prepared at different times during the life cycle of the fungus indicate that the increase in cAMP-binding activity occurring during sporulation, as well as its decrease during germination, are associated with the accumulation of the regulatory subunit during sporulation and its disappearance during germination, respectively. Pulse labeling with [35S]methionine and immunoprecipitation indicate that the accumulation of R is due to its increased synthesis during sporulation. Two-dimensional gel electrophoresis of affinity purified cell extracts obtained after [35S]methionine pulse labeling during sporulation confirms de novo synthesis of R during this stage and furthermore shows that the protein is rapidly phosphorylated after its synthesis. In vitro translation studies using RNA isolated from different stages of the life cycle followed by immunoprecipitation have shown that the time course of expression of the mRNA coding for the regulatory subunit parallels the rate of its synthesis in vivo.

cAMP-dependent protein kinase from Dictyostelium discoideum

The cAMP-dependent protein kinase (cAK) from Dictyostelium discoideum is an enzyme composed of one catalytic and one regulatory subunit. Upon binding of cAMP, the holoenzyme dissociates to liberate free active catalytic subunits. The cAK is developmentally regulated, ranging from very little activity in vegetative cells to maximal expression in postaggregative cells. Although there is no immunological cross-reaction between the subunits of cAKs from Dictyostelium and from other organisms, they share several biochemical properties. A complete cDNA for the regulatory subunit has been cloned and sequenced. Only one copy of the gene for the regulatory subunit is present per haploid genome. On the basis of the comparison of the structure of the cAK from Dictyostelium with its counterparts in yeast and higher eukaryotes, we propose a model for the evolution of cyclic-nucleotide-binding proteins.

Translocation of an unusual cAMP receptor to the nucleus during development of Dictyostelium discoideum

cAMP has been implicated in the control of the expression of developmental genes in Dictyostelium discoideum. To determine the potential role of cAMP receptors as regulators of gene expression, we have used immunocytochemical and immunoblotting techniques to reveal the subcellular localization of a cAMP binding protein CABP1. Most of the CABP1 antigen in early developing cells is localized near the cell periphery, with a small amount found in the nucleus. The level of CABP1 in the nucleus increases approximately 30-fold during development. Moreover, immunofluorescence studies showed that CABP1 can also be detected on the cell surface. Binding of anti-CABP1 to intact cells followed by reaction with 125I-labeled secondary antibody revealed that the cell-surface CABP1 activity peaks during aggregation and culmination. In addition, several proteins related to CABP1 are found mainly in the nuclear fraction of developing cells. The possible role of these proteins in the regulation of developmental gene activity is discussed.

Cloning and cDNA sequence of the regulatory subunit of cAMP-dependent protein kinase from Dictyostelium discoideum

cDNA clones encoding the regulatory subunit of the cAMP-dependent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37) from Dictyostelium discoideum were isolated by immunoscreening of a cDNA library constructed in the expression vector lambda gt11. High-affinity cAMP-binding activity was detected in extracts from bacteria lysogenized with these clones. Nucleotide sequence analysis of three overlapping clones allowed the determination of a 1195-base-pair cDNA sequence coding for the entire regulatory subunit and containing nontranslated 5' and 3' sequences. The open reading frame codes for a protein of 327 amino acids, with molecular weight 36,794. The regulatory subunit from Dictyostelium shares a high degree of homology with its mammalian counterparts, but is lacking the NH2-terminal domain required for the association of regulatory subunits into dimers in other eukaryotes. On the basis of the comparison of the regulatory subunits from Dictyostelium, yeast, and bovine tissues, a model for the evolution of these proteins is proposed.

cAMP induction of prespore and prestalk gene expression in Dictyostelium is mediated by the cell-surface cAMP receptor

Extracellular adenosine 3',5'-cyclic monophosphate (cAMP) is required for cell-type-specific gene expression in developing Dictyostelium discoideum. We have developed a microassay for the expression of these genes, using antibodies directed against their protein products. To characterize the transduction mechanism, we have used in this assay cAMP analogues that preferentially activate either the cell-surface cAMP receptor or the internal cAMP-dependent protein kinase. N6-(aminohexyl) cAMP activates the Dictyostelium cAMP-dependent protein kinase but does not bind to the cell-surface cAMP receptor and does not cause cell-type-specific gene expression. 2'-Deoxy-cAMP does not activate the cAMP-dependent protein kinase but binds to the receptor and causes cell-type-specific gene expression. Cyclic AMP-induced accumulation of prestalk mRNA in shaking cultures still occurs in the presence of caffeine, which blocks the receptor-coupled activation of adenyl cyclase. This suggests that the extracellular cAMP induction of cell-type-specific gene expression in developing Dictyostelium cells is mediated by the cell-surface cAMP receptor and that activating adenyl cyclase by this receptor is not essential. Using the N6-(aminohexyl) cAMP to competitively inhibit phosphodiesterase, we show that 30 nM cAMP is sufficient to induce prestalk or prespore gene expression.

Comparison of the regulatory and catalytic subunits of cAMP dependent protein kinase from Dictyostelium discoideum and bovine heart using polyclonal antibodies

The purified regulatory (R) and catalytic (C) subunits of cAMP dependent protein kinase (cAK) from the primitive eukaryote Dictyostelium discoideum have been compared with the homologous proteins from bovine heart by SDS-PAGE followed by Western blotting using polyclonal antibodies. No cross-reaction could be demonstrated by this technique although the slime mold subunits share several functional properties with their mammalian counterparts and are able to form functional hybrid holoenzymes.