The protein kinase A catalytic subunit Cbeta2: molecular characterization and distribution of the splice variant

Evolution of the cAMP-dependent protein kinase (PKA) catalytic subunit isoforms

The 3’,5’-cyclic adenosine monophosphate (cAMP)-dependent protein kinase, or protein kinase A (PKA), pathway is one of the most versatile and best studied signaling pathways in eukaryotic cells. The two paralogous PKA catalytic subunits Cα and Cβ, encoded by the genes PRKACA and PRKACB, respectively, are among the best understood model kinases in signal transduction research. In this work, we explore and elucidate the evolution of the alternative 5’ exons and the splicing pattern giving rise to the numerous PKA catalytic subunit isoforms. In addition to the universally conserved Cα1/Cβ1 isoforms, we find kinase variants with short N-termini in all main vertebrate classes, including the sperm-specific Cα2 isoform found to be conserved in all mammals. We also describe, for the first time, a PKA Cα isoform with a long N-terminus, paralogous to the PKA Cβ2 N-terminus. An analysis of isoform-specific variation highlights residues and motifs that are likely to be of functional importance.

Human heart failure is accompanied by altered protein kinase A subunit expression and post-translational state

β-Adrenergic receptor blockade reduces total mortality and all-cause hospitalizations in patients with heart failure (HF). Nonetheless, β-blockade does not halt disease progression, suggesting that cAMP-dependent protein kinase (PKA) signaling downstream of β-adrenergic receptor activation may persist through unique post-translational states. In this study, human myocardial tissue was used to examine the state of PKA subunits. As expected, total myosin binding protein-C phosphorylation and Ser23/24 troponin I phosphorylation significantly decreased in HF. Examination of PKA subunits demonstrated no change in type II regulatory (RIIα) or catalytic (Cα) subunit expression, although site specific RIIα (Ser96) and Cα (Thr197) phosphorylation were increased in HF. Further, the expression of type I regulatory subunit (RI) was increased in HF. Isoelectric focusing of RIα demonstrated up to three variants, consistent with reports that Ser77 and Ser83 are in vivo phosphorylation sites. Western blots with site-specific monoclonal antibodies showed increased Ser83 phosphorylation in HF. 8-fluo-cAMP binding by wild type and phosphomimic Ser77 and Ser83 mutant RIα proteins demonstrated reduced Kd for the double mutant as compared to WT RIα. Therefore, failing myocardium displays altered expression and post-translational modification of PKA subunits that may impact downstream signaling.

Computational prediction and characterisation of ubiquitously expressed new splice variant of Prkaca gene in mouse

Prkaca gene of mouse encodes for a cAMP dependent protein kinase catalytic alpha subunit. PKA occurs naturally as a 4-membered structure having two regulatory (R) and two catalytic (C) subunits each encoded by separate gene. Alternatively spliced two transcript variants are known for the Prkaca gene, which encode for two isoforms of PKA C-subunits, namely Cα1 and Cα2. These isoforms arise as a result of alternative splicing of the first coding exon with the internal exons. We have identified a new transcript variant using combinatorial approach of bioinformatics and molecular biology techniques involving RT-PCR, semi-nested PCR and sequencing. The new transcript variant encoding Cα3 isoform has N-terminus that differs from Cα1 and Cα2 isoforms. Cα3 isoform also arise as a result of alternative splicing of first coding exon with the internal exon. Newly identified transcript is expressed ubiquitously in different tissues examined.

The testis-specific Cα2 subunit of PKA is kinetically indistinguishable from the common Cα1 subunit of PKA

Background

The two variants of the α-form of the catalytic (C) subunit of protein kinase A (PKA), designated Cα1 and Cα2, are encoded by the PRKACA gene. Whereas Cα1 is ubiquitous, Cα2 expression is restricted to the sperm cell. Cα1 and Cα2 are encoded with different N-terminal domains. In Cα1 but not Cα2 the N-terminal end introduces three sites for posttranslational modifications which include myristylation at Gly1, Asp-specific deamidation at Asn2 and autophosphorylation at Ser10. Previous reports have implicated specific biological features correlating with these modifications on Cα1. Since Cα2 is not modified in the same way as Cα1 we tested if they have distinct biochemical activities that may be reflected in different biological properties.

Results

We show that Cα2 interacts with the two major forms of the regulatory subunit (R) of PKA, RI and RII, to form cAMP-sensitive PKAI and PKAII holoenzymes both in vitro and in vivo as is also the case with Cα1. Moreover, using Surface Plasmon Resonance (SPR), we show that the interaction patterns of the physiological inhibitors RI, RII and PKI were comparable for Cα2 and Cα1. This is also the case for their potency to inhibit catalytic activities of Cα2 and Cα1.

Conclusion

We conclude that the regulatory complexes formed with either Cα1 or Cα2, respectively, are indistinguishable.

Identification, cloning and characterization of a novel 47 kDa murine PKA C subunit homologous to human and bovine Cbeta2

Background

Two main genes encoding the catalytic subunits Cα and Cβ of cyclic AMP dependent protein kinase (PKA) have been identified in all vertebrates examined. The murine, bovine and human Cβ genes encode several splice variants, including the splice variant Cβ2. In mouse Cβ2 has a relative molecular mass of 38 kDa and is only expressed in the brain. In human and bovine Cβ2 has a relative molecular mass of 47 kDa and is mainly expressed in lymphoid tissues.

Results

We identified a novel 47 kDa splice variant encoded by the mouse Cβ gene that is highly expressed in lymphoid cells. Cloning, expression, and production of a sequence-specific antiserum and characterization of PKA catalytic subunit activities demonstrated the 47 kDa protein to be a catalytically active murine homologue of human and bovine Cβ2. Based on the present results and the existence of a human brain-specifically expressed Cβ splice variant designated Cβ4 that is identical to the former mouse Cβ2 splice variant, the mouse splice variant has now been renamed mouse Cβ4.

Conclusion

Murine lymphoid tissues express a protein that is a homologue of human and bovine Cβ2. The murine Cβ gene encodes the splice variants Cβ1, Cβ2, Cβ3 and Cβ4, as is the case with the human Cβ gene.

Multiple independent kinase cascades are targeted by hyperosmotic stress but only one activates stress kinase p38

In this report, we analyse the effects of osmotic shock on signal transduction in CHO cells. We demonstrate that at least three different kinase cascades are switched on upon osmotic shock, namely PKA, AMPK, and MLTK. Whereas PKA from cells treated with forskolin activated stress kinase p38, PKA from cells treated with sorbitol did not activate p38, although the enzyme is activated in both cases as analysed in vitro using a specific peptide target. Further, osmolar shock activated AMPK but treatment of the cells with the AMPK activator 5-amino-4-imidazolecarboxamide (AICAr) did not result in p38 activation, strongly suggesting that AMPK is not involved in stress kinase activation. Transfection of CHO cells with dominant negative recombinants of MLTKalpha resulted in inhibition of sorbitol-mediated p38 activation, indicating that the mixed-lineage kinase is involved in the activation of p38 by sorbitol. Finally, in CHO cells overexpressing wild-type MLTKalpha, no activation of AMPK of PKA could be demonstrated, indicating that the activated kinase cascades are not involved in a cross-talk process.

Identification of novel splice variants of the human catalytic subunit Cbeta of cAMP-dependent protein kinase

Four different isoforms of the catalytic subunit of cAMP-dependent protein kinase, termed Calpha, Cbeta, Cgamma and PrKX have been identified. Here we demonstrate that the human Cbeta gene encodes six splice variants, designated Cbeta1, Cbeta2, Cbeta3, Cbeta4, Cbeta4ab and Cbeta4abc. The Cbeta splice variants differ in their N-terminal ends due to differential splicing of four different forms of exon 1 designated exon 1-1, 1-2, 1-3, 1-4 and three exons designated a, b and c. All these exons are located upstream of exon 2 in the Cbeta gene. The previously identified human Cbeta variant has been termed Cbeta1, and is similar to the Cbeta isoform identified in the mouse, ox, pig and several other mammals. Human Cbeta2, which is the homologue of bovine Cbeta2, has no homologue in the mouse. Human Cbeta3 and Cbeta4 are homologous to the murine Cbeta3 and Cbeta2 splice variants, whereas human Cbeta4ab and Cbeta4abc represent novel isofoms previously not identified in any other species. At the mRNA level, the Cbeta splice variants reveal tissue specific expression. Cbeta1 was most abundantly expressed in the brain, with low-level expression in several other tissues. The Cbeta3 and Cbeta4 splice variants were uniquely expressed in human brain in contrast to Cbeta2, which was most abundantly expressed in tissues of the immune system, with no detectable expression in brain. We suggest that the various Cbeta splice variants when complexed with regulatory subunits may give rise to novel holoenzymes of protein kinase A that may be important for mediating specific effects of cAMP.

Genomic organization and tissue-specific expression of splice variants of mouse organic anion transporting polypeptide 2

cDNAs that code for mouse organic anion transporting polypeptide 2 (oatp2) have been cloned. At least three forms of mouse oatp2 cDNAs containing the same coding sequence were isolated. The common coding sequence is for a protein of 670 amino acids with 12 putative transmembrane domains. The deduced amino acid sequence of the mouse oatp2 shares 89% identity with the reported rat oatp2. Cloning and analysis of mouse oatp2 gene indicates that these isoforms are alternatively spliced products from the same gene. Heterogeneity was observed in the 5'-untranslated region of the cDNAs. Two of the three isoforms lacked the noncoding exon 3 sequence. Northern-blot hybridization analysis using the exon 3-specific probes demonstrated that mouse oatp2 mRNA containing exon 3 sequence is expressed in heart and lung, whereas exon 1-, 2-, and 17-specific probes detected mRNA only in brain and liver. The mouse oatp2 gene consists of 17 exons, including three noncoding exons, and 16 introns. All of the introns are flanked by GT-AG splice sequences except for intron 10 that is flanked by GC-AG splice sequence.