Single amino acids determine specificity of binding of protein kinase A regulatory subunits by protein kinase A anchoring proteins

Structure and Composition of Spermatozoa Fibrous Sheath in Diverse Groups of Metazoa

The proper functioning and assembly of the sperm flagella structures contribute significantly to spermatozoa motility and overall male fertility. However, the fine mechanisms of assembly steps are poorly studied due to the high diversity of cell types, low solubility of the corresponding protein structures, and high tissue and cell specificity. One of the open questions for investigation is the attachment of longitudinal columns to the doublets 3 and 8 of axonemal microtubules through the outer dense fibers. A number of mutations affecting the assembly of flagella in model organisms are known. Additionally, evolutionary genomics data and comparative analysis of flagella morphology are available for a set of non-model species. This review is devoted to the analysis of diverse ultrastructures of sperm flagellum of Metazoa combined with an overview of the evolutionary distribution and function of the mammalian fibrous sheath proteins.

AKAP3-mediated type I PKA signaling is required for mouse sperm hyperactivation and fertility†

The protein kinase A (PKA) signaling pathway, which mediates protein phosphorylation, is important for sperm motility and male fertility. This process relies on A-kinase anchoring proteins that organize PKA and its signalosomes within specific subcellular compartments. Previously, it was found that the absence of A-kinase anchoring protein 3 (AKAP3) leads to multiple morphological abnormalities in mouse sperm. But how AKAP3 regulates sperm motility is yet to be elucidated. AKAP3 has two amphipathic domains, here named dual and RI, in its N-terminus. These domains are responsible for binding regulatory subunits I alpha (RIα) and II alpha (RIIα) of PKA and for RIα only, respectively. Here, we generated mutant mice lacking the dual and RI domains of AKAP3. It was found that the deletion of these domains caused male mouse infertile, accompanied by mild defects in the fibrous sheath of sperm tails. Additionally, the levels of serine/threonine phosphorylation of PKA substrates and tyrosine phosphorylation decreased in the mutant sperm, which exhibited a defect in hyperactivation under capacitation conditions. The protein levels of PKA subunits remained unchanged. But, interestingly, the regulatory subunit RIα was mis-localized from principal piece to midpiece of sperm tail, whereas this was not observed for RIIα. Further protein-protein interaction assays revealed a preference for AKAP3 to bind RIα over RIIα. Collectively, our findings suggest that AKAP3 is important for sperm hyperactivity by regulating type-I PKA signaling pathway mediated protein phosphorylation via its dual and RI domains.

Phosphorylation, compartmentalization, and cardiac function

Protein phosphorylation is a fundamental element of cell signaling. First discovered as a biochemical switch in glycogen metabolism, we now know that this posttranslational modification permeates all aspects of cellular behavior. In humans, over 540 protein kinases attach phosphate to acceptor amino acids, whereas around 160 phosphoprotein phosphatases remove phosphate to terminate signaling. Aberrant phosphorylation underlies disease, and kinase inhibitor drugs are increasingly used clinically as targeted therapies. Specificity in protein phosphorylation is achieved in part because kinases and phosphatases are spatially organized inside cells. A prototypic example is compartmentalization of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A through association with A-kinase anchoring proteins. This configuration creates autonomous signaling islands where the anchored kinase is constrained in proximity to activators, effectors, and selected substates. This article primarily focuses on A kinase anchoring protein (AKAP) signaling in the heart with an emphasis on anchoring proteins that spatiotemporally coordinate excitation-contraction coupling and hypertrophic responses.

Quantitative phosphoproteomics analyses reveal the regulatory mechanisms related to frozen-thawed sperm capacitation and acrosome reaction in yak (Bos grunniens)

Mammalian spermatozoa are not mature after ejaculation and must undergo additional functional and structural changes within female reproductive tracts to achieve subsequent fertilization, including both capacitation and acrosome reaction (AR), which are dominated by post-translational modifications (PTMs), especially phosphorylation. However, the mechanism of protein phosphorylation during frozen-thawed sperm capacitation and AR has not been well studied. In this study, the phosphoproteomics approach was employed based on tandem mass tag (TMT) labeling combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) strategy to analyze frozen-thawed sperm in Ashidan yak under three sequential conditions (density gradient centrifugation-based purification, incubation in the capacitation medium and induction of AR processes by the calcium ionophore A23187 treatment). The identification of 1,377 proteins with 5,509 phosphorylation sites revealed changes in phosphorylation levels of sperm-specific proteins involved in regulation of spermatogenesis, sperm motility, energy metabolism, cilium movement, capacitation and AR. Some phosphorylated proteins, such as AKAP3, AKAP4, SPA17, PDMD11, CABYR, PRKAR1A, and PRKAR2A were found to regulate yak sperm capacitation and AR though the cAMP/PKA signaling pathway cascades. Notably, the phosphorylation level of SPA17 at Y156 increased in capacitated sperm, suggesting that it is also a novel functional protein besides AKAPs during sperm capacitation. Furthermore, the results of this study suggested that the phosphorylation of PRKAR1A and PRKAR2A, and the dephosphorylation of CABYR both play key regulatory role in yak sperm AR process. Protein-protein interaction analysis revealed that differentially phosphorylated proteins (AKAP3, AKAP4, FSIP2, PSMD11, CABYR, and TPPP2) related to capacitation and AR process played a key role in protein kinase A binding, sperm motility, reproductive process, cytoskeleton and sperm flagella function. Taken together, these data provide not only a solid foundation for further exploring phosphoproteome of sperm in yak, but an efficient way to identify sperm fertility-related marker phosphorylated proteins.

Implications of critical node-dependent unidirectional cross-talk of Plasmodium SUMO pathway proteins

The endoparasitic pathogen, Plasmodium falciparum (Pf), modulates protein-protein interactions to employ post-translational modifications like SUMOylation to establish successful infections. The interaction between E1 and E2 (Ubc9) enzymes governs species specificity in the Plasmodium SUMOylation pathway. Here, we demonstrate that a unidirectional cross-species interaction exists between Pf-SUMO and human E2, whereas Hs-SUMO1 failed to interact with Pf-E2. Biochemical and biophysical analyses revealed that surface-accessible aspartates of Pf-SUMO determine the efficacy and specificity of SUMO-Ubc9 interactions. Furthermore, we demonstrate that critical residues of the Pf-Ubc9 N terminus are responsible for diminished Hs-SUMO1 and Pf-Ubc9 interaction. Mutating these residues to corresponding Hs-Ubc9 residues restores electrostatic, π-π, and hydrophobic interactions and allows efficient cross-species interactions. We suggest that, in comparison with human counterparts, Plasmodium SUMO and Ubc9 proteins have acquired critical changes on their surfaces as nodes, which Plasmodium can use to exploit the host SUMOylation machinery.

Effects of Calyculin a on the Motility and Protein Phosphorylation in Frozen-Thawed Bull Spermatozoa

In this study, we examined the effects of calyculin A, a phosphatase inhibitor, on motility, protein phosphorylation, and the distribution of phospho-(Ser/Thr) PKA substrates in frozen-thawed bull spermatozoa that are actually used by most farmers for breeding. The data showed that calyculin A, which has been reported to have a positive effect on the motility of ejaculated fresh spermatozoa, distinctly decreased the motility of frozen-thawed bull spermatozoa even if a cell activator, such as caffeine, was present in the incubation medium and that the suppressive effect of calyculin A was dose-dependent and continued for at least 200 min. Immunoblot analyses revealed that de novo protein phosphorylation was not detected in spermatozoa exposed to caffeine or dbcAMP (a cell-permeable cAMP analog), while the addition of calyculin A to the medium brought about the appearance of several phosphorylated proteins at 50 kDa and 75 kDa, suggesting that 50 kDa and 75 kDa proteins, which were phosphorylated by activation of cAMP-dependent PKA, were not dephosphorylated and were accumulated in spermatozoa due to the suppression of calyculin A-sensitive protein phosphatases. Immunofluorescence microscopy revealed that calyculin A caused, alone or in conjunction with caffeine or dbcAMP, the accumulation of phospho-PKA substrates at the annulus, although caffeine or dbcAMP alone did not. This study suggested that calyculin A decreases the motility of frozen-thawed bull spermatozoa concomitant with the accumulation of phospho-(Ser/Thr) PKA substrates at the annulus of flagella.

Comparative Proteomics and Phosphoproteomics Analysis Reveal the Possible Breed Difference in Yorkshire and Duroc Boar Spermatozoa

Sperm cells are of unique elongated structure and function, the development of which is tightly regulated by the existing proteins and the posttranslational modifications (PTM) of these proteins. Based on the phylogenetic relationships of various swine breeds, Yorkshire boar is believed to be distinctly different from Duroc boar. The comprehensive differential proteomics and phosphoproteomics profilings were performed on spermatozoa from both Yorkshire and Duroc boars. By both peptide and PTM peptide quantification followed by statistical analyses, 167 differentially expressed proteins were identified from 1,745 proteins, and 283 differentially expressed phosphopeptides corresponding to 102 unique differentially phosphorylated proteins were measured from 1,140 identified phosphopeptides derived from 363 phosphorylated proteins. The representative results were validated by Western blots. Pathway enrichment analyses revealed that majority of differential expression proteins and differential phosphorylation proteins were primarily concerned with spermatogenesis, male gamete generation, sperm motility, energy metabolism, cilium morphogenesis, axonemal dynein complex assembly, sperm–egg recognition, and capacitation. Remarkably, axonemal dynein complex assembly related proteins, such as SMCP, SUN5, ODF1, AKAP3, and AKAP4 that play a key regulatory role in the sperm physiological functions, were significantly higher in Duroc spermatozoa than that of Yorkshire. Furthermore, phosphorylation of sperm-specific proteins, such as CABYR, ROPN1, CALM1, PRKAR2A, and PRKAR1A, participates in regulation of the boar sperm motility mainly through the cAMP/PKA signal pathway in different breeds, demonstrating that protein phosphorylation may be an important mechanism underlying the sperm diversity. Protein–protein interaction analysis revealed that the 14 overlapped proteins between differential expression proteins and differential phosphorylation proteins potentially played a key role in sperm development and motility of the flagellum, including the proteins ODF1, SMCP, AKAP4, FSIP2, and SUN5. Taken together, these physiologically and functionally differentially expressed proteins (DEPs) and differentially expressed phosphorylated proteins (DPPs) may constitute the proteomic backgrounds between the two different boar breeds. The validation will be performed to delineate the roles of these PTM proteins as modulators of Yorkshire and Duroc boar spermatozoa.

A Kinase Anchor Protein 4 Is Vulnerable to Oxidative Adduction in Male Germ Cells

Oxidative stress is a leading causative agent in the defective sperm function associated with male infertility. Such stress commonly manifests via the accumulation of pathological levels of the electrophilic aldehyde, 4-hydroxynonenal (4HNE), generated as a result of lipid peroxidation. This highly reactive lipid aldehyde elicits a spectrum of cytotoxic lesions owing to its propensity to form stable adducts with biomolecules. Notably however, not all elements of the sperm proteome appear to display an equivalent vulnerability to 4HNE modification, with only a small number of putative targets having been identified to date. Here, we validate one such target of 4HNE adduction, A-Kinase Anchor Protein 4 (AKAP4); a major component of the sperm fibrous sheath responsible for regulating the signal transduction and metabolic pathways that support sperm motility and capacitation. Our data confirm that both the precursor (proAKAP4), and mature form of AKAP4, are conserved targets of 4HNE adduction in primary cultures of post-meiotic male germ cells (round spermatids) and in mature mouse and human spermatozoa. We further demonstrate that 4HNE treatment of round spermatids and mature spermatozoa results in a substantial reduction in the levels of both proAKAP4 and AKAP4 proteins. This response proved refractory to pharmacological inhibition of proteolysis, but coincided with an apparent increase in the degree of protein aggregation. Further, we demonstrate that 4HNE-mediated protein degradation and/or aggregation culminates in reduced levels of capacitation-associated phosphorylation in mature human spermatozoa, possibly due to dysregulation of the signaling framework assembled around the AKAP4 scaffold. Together, these findings suggest that AKAP4 plays an important role in the pathophysiological responses to 4HNE, thus strengthening the importance of AKAP4 as a biomarker of sperm quality, and providing the impetus for the design of an efficacious antioxidant-based intervention strategy to alleviate sperm dysfunction.

Proteomic analysis of mature and immature ejaculated spermatozoa from fertile men

Dysfunctional spermatozoa maturation is the main reason for the decrease in sperm motility and morphology in infertile men. Ejaculated spermatozoa from healthy fertile men were separated into four fractions using three-layer density gradient. Proteins were extracted and bands were digested on a LTQ-Orbitrap Elite hybrid mass spectrometer system. Functional annotations of proteins were obtained using bioinformatics tools and pathway databases. Western blotting was performed to verify the expression levels of the proteins of interest. 1469 proteins were identified in four fractions of spermatozoa. The number of detected proteins decreased according to the maturation level of spermatozoa. During spermatozoa maturation, proteins involved in gamete generation, cell motility, energy metabolism and oxidative phosphorylation processes showed increasing expression levels and those involved in protein biosynthesis, protein transport, protein ubiquitination, and response to oxidative stress processes showed decreasing expression levels. We validated four proteins (HSP 70 1A, clusterin, tektin 2 and tektin 3) by Western blotting. The study shows protein markers that may provide insight into the ejaculated spermatozoa proteins in different stages of sperm maturation that may be altered or modified in infertile men.

Translocation of Tektin 3 to the equatorial segment of heads in bull spermatozoa exposed to dibutyryl cAMP and calyculin A

Tektins (TEKTs) are filamentous proteins associated with microtubules in cilia, flagella, basal bodies, and centrioles. Five TEKTs (TEKT1, -2, -3, -4, and -5) have been identified as components of mammalian sperm flagella. We previously reported that TKET1 and -3 are also present in the heads of rodent spermatozoa. The present study clearly demonstrates that TEKT2 is present at the acrosome cap whereas TEKT3 resides just beneath the plasma membrane of the post-acrosomal region of sperm heads in unactivated bull spermatozoa, and builds on the distributional differences of TEKT1, -2, and -3 on sperm heads. We also discovered that hyperactivation of bull spermatozoa by cell-permeable cAMP and calyculin A, a protein phosphatase inhibitor, promoted translocation of TEKT3 from the post-acrosomal region to the equatorial segment in sperm heads, and that TEKT3 accumulated at the equatorial segment is lost upon acrosome reaction. Thus, translocation of TEKT3 to the equatorial segment may be a capacitation- or hyperactivation-associated phenomenon in bull spermatozoa. Mol. Reprod. Dev. 84: 30-43, 2017. © 2016 Wiley Periodicals, Inc.

A-Kinase Anchoring Protein 4 (AKAP4) is an ERK1/2 substrate and a switch molecule between cAMP/PKA and PKC/ERK1/2 in human spermatozoa

Mammalian spermatozoa undergo capacitation and acrosome reaction in order to fertilize the egg. The PKC-ERK1/2 pathway plays an important role in human spermatozoa motility, capacitation and the acrosome reaction. Here we demonstrate that ERK1/2 phosphorylates proAKAP4 on Thr265 in human spermatozoa in vitro and in vivo. Cyclic AMP (cAMP) had no effect on ERK1/2 activity in human spermatozoa, but stimulated the MAPK in mouse pituitary LβT2 gonadotrope cells. cAMP via PKA attenuates PKC-dependent ERK1/2 activation only in the presence of proAKAP4. St-HT31, which disrupts PKA-regulatory subunit II (PKA-RII) binding to AKAP abrogates the inhibitory effect of cAMP in human spermatozoa and in HEK293T cells expressing proAKAP4. In transfected HEK293T cells, PMA relocated proAKAP4, but not proAKAP4-T265A to the Golgi in an ERK1/2-dependnet manner. Similarly, AKAP4 is localized to the spermatozoa principal piece and is relocated to the mid-piece and the postacrosomal region by PMA. Furthermore, using capacitated sperm we found that cAMP reduced PMA-induced ERK1/2 activation and acrosome reaction. Thus, the physiological role of the negative crosstalk between the cAMP/PKA/AKAP4 and the PKC/ERK1/2 pathways is to regulate capacitation and acrosome reaction.

Molecular Cloning of Spergen-4, Encoding a Spermatogenic Cell-Specific Protein Associated with Sperm Flagella and the Acrosome Region in Rat Spermatozoa

We used a differential display in combination with complementary DNA (cDNA) cloning approach to isolate a novel rat gene LOC690919 with an open reading frame of 1227-length nucleotides encoding a protein of 409 amino acids. This gene was designated as Spergen-4 (a spermatogenic cell-specific gene-4). Spergen-4 mRNA was highly expressed in testis, and its expression was detected in rat testis starting at three weeks of postnatal development and persisting up to adulthood. Mouse and human orthologs, which lack N-terminal 77 amino acid residues of rat Spegen-4, were found in the database. Immunofluorescence microscopy and immunoblot analysis demonstrated that Spergen-4 was not expressed in spermatogonia, spermatocytes, and round spermatids, but was restrictedly detected at sperm head, cytoplasm, and developing flagella of elongated spermatids in rat testis. In mature spermatozoa, Spergen-4 was detected at the acrosome region as well as the principal piece of flagella. Spergen-4 immunosignal disappeared from sperm heads on acrosome reaction induced by progesterone. These data suggest that Spergen-4 integrated into elongated spermatids during spermiogenesis serves as a constituent for acrosome region and flagella of rat spermatozoa.

Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling

The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer.

Molecular cloning and subcellular localization of Tektin2-binding protein 1 (Ccdc 172) in rat spermatozoa

Tektins (TEKTs) are composed of a family of filament-forming proteins localized in cilia and flagella. Five types of mammalian TEKTs have been reported, all of which have been verified to be present in sperm flagella. TEKT2, which is indispensable for sperm structure, mobility, and fertilization, was present at the periphery of the outer dense fiber (ODF) in the sperm flagella. By yeast two-hybrid screening, we intended to isolate flagellar proteins that could interact with TEKT2, which resulted in the isolation of novel two genes from the mouse testis library, referred as a TEKT2-binding protein 1 (TEKT2BP1) and -protein 2 (TEKT2BP2). In this study, we characterized TEKT2BP1, which is registered as a coiled-coil domain-containing protein 172 (Ccdc172) in the latest database. RT-PCR analysis indicated that TEKT2BP1 was predominantly expressed in rat testis and that its expression was increased after 3 weeks of postnatal development. Immunocytochemical studies discovered that TEKT2BP1 localized in the middle piece of rat spermatozoa, predominantly concentrated at the mitochondria sheath of the flagella. We hypothesize that the TEKT2-TEKT2BP1 complex might be involved in the structural linkage between the ODF and mitochondria in the middle piece of the sperm flagella.

Male mice express spermatogenic cell-specific triosephosphate isomerase isozymes

Triosephosphate isomerase 1 (TPI1) is a member of the glycolytic pathway, which is a critical source of energy for motility in mouse sperm. By immunoblotting, we detected two male, germ line-specific TPI1 bands (M_r 33,400 and 30,800) as well as the somatic-type band (M_r 27,700). Although all three bands were observed in spermatogenic cells, somatic-type TPI1 disappeared from sperm during epididymal maturation. In vitro dephosphorylation analysis suggested that the two male, germ line-specific TPI1 bands were not the result of phosphorylation of the 27,700 M_r TPI1 band. The M_r 33,400; 30,800; and 27,700 TPI1 bands corresponded to the respective sizes of the proteins predicted to use the first, second, and third possible initiation codons of the Tpi1 cDNA. We performed immunofluorescence on epididymal sperm and determined that TPI1 specifically localized in the principal piece. The antibody staining was stronger in cauda epididymal sperm than in caput epididymal sperm, a finding consistent with the identification of TPI1 as a cauda epididymal sperm-enriched protein. Immunofluorescence with sodium dodecyl sulfate (SDS)-insoluble flagellar accessory structures showed a strong TPI1 signal only in the principal piece, indicating that TPI1 is a component of the fibrous sheath. Northern blot hybridization detected longer Tpi1 transcripts (1.56 kb) in mouse testis, whereas somatic tissues had shorter transcripts (1.32 kb). As there is only one triosephosphate isomerase gene in the mouse genome, we conclude that the three variants we see in sperm result from the use of alternative translation start codons in spermatogenic cells. Mol. Reprod. Dev. 80: 862–870, 2013. © 2013 The Authors. Published by Wiley Periodicals, Inc. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

PKA and GAB2 play central roles in the FSH signaling pathway to PI3K and AKT in ovarian granulosa cells

Controlled maturation of ovarian follicles is necessary for fertility. Follicles are restrained at an immature stage until stimulated by FSH secreted by pituitary gonadotropes. FSH acts on granulosa cells within the immature follicle to inhibit apoptosis, promote proliferation, stimulate production of steroid and protein hormones, and induce ligand receptors and signaling intermediates. The phosphoinositide 3-kinase (PI3K)/AKT (protein kinase B) pathway is a pivotal signaling corridor necessary for transducing the FSH signal. We report that protein kinase A (PKA) mediates the actions of FSH by signaling through multiple targets to activate PI3K/AKT. PKA uses a route that promotes phosphorylation of insulin receptor substrate-1 (IRS-1) on Tyr(989), a canonical binding site for the 85-kDa regulatory subunit of PI3K that allosterically activates the catalytic subunit. PI3K activation leads to activation of AKT through phosphorylation of AKT on Thr(308) and Ser(473). The adaptor growth factor receptor bound protein 2-associated binding protein 2 (GAB2) is present in a preformed complex with PI3K heterodimer and IRS-1, it is an A-kinase anchoring protein that binds the type I regulatory subunit of PKA, and it is phosphorylated by PKA on Ser(159). Overexpression of GAB2 enhances FSH-stimulated AKT phosphorylation. GAB2, thus, seems to coordinate signals from the FSH-stimulated rise in cAMP that leads to activation of PI3K/AKT. The ability of PKA to commandeer IRS-1 and GAB2, adaptors that normally integrate receptor/nonreceptor tyrosine kinase signaling into PI3K/AKT, reveals a previously unrecognized route for PKA to activate a pathway that promotes proliferation, inhibits apoptosis, enhances translation, and initiates differentiation of granulosa cells.

Molecular evolution of A-kinase anchoring protein (AKAP)-7: implications in comparative PKA compartmentalization

Background

A-Kinase Anchoring Proteins (AKAPs) are molecular scaffolding proteins mediating the assembly of multi-protein complexes containing cAMP-dependent protein kinase A (PKA), directing the kinase in discrete subcellular locations. Splice variants from the AKAP7 gene (AKAP15/18) are vital components of neuronal and cardiac phosphatase complexes, ion channels, cardiac Ca²⁺ handling and renal water transport.

Results

Shown in evolutionary analyses, the formation of the AKAP7-RI/RII binding domain (required for AKAP/PKA-R interaction) corresponds to vertebrate-specific gene duplication events in the PKA-RI/RII subunits. Species analyses of AKAP7 splice variants shows the ancestral AKAP7 splice variant is AKAP7α, while the ancestral long form AKAP7 splice variant is AKAP7γ. Multi-species AKAP7 gene alignments, show the recent formation of AKAP7δ occurs with the loss of native AKAP7γ in rats and basal primates. AKAP7 gene alignments and two dimensional Western analyses indicate that AKAP7γ is produced from an internal translation-start site that is present in the AKAP7δ cDNA of mice and humans but absent in rats. Immunofluorescence analysis of AKAP7 protein localization in both rat and mouse heart suggests AKAP7γ replaces AKAP7δ at the cardiac sarcoplasmic reticulum in species other than rat. DNA sequencing identified Human AKAP7δ insertion-deletions (indels) that promote the production of AKAP7γ instead of AKAP7δ.

Conclusions

This AKAP7 molecular evolution study shows that these vital scaffolding proteins developed in ancestral vertebrates and that independent mutations in the AKAP7 genes of rodents and early primates has resulted in the recent formation of AKAP7δ, a splice variant of likely lesser importance in humans than currently described.

Localization and quaternary structure of the PKA RIβ holoenzyme

Specificity for signaling by cAMP-dependent protein kinase (PKA) is achieved by both targeting and isoform diversity. The inactive PKA holoenzyme has two catalytic (C) subunits and a regulatory (R) subunit dimer (R(2):C(2)). Although the RIα, RIIα, and RIIβ isoforms are well studied, little is known about RIβ. We show here that RIβ is enriched selectively in mitochondria and hypothesized that its unique biological importance and functional nonredundancy will correlate with its structure. Small-angle X-ray scattering showed that the overall shape of RIβ(2):C(2) is different from its closest homolog, RIα(2):C(2). The full-length RIβ(2):C(2) crystal structure allows us to visualize all the domains of the PKA holoenzyme complex and shows how isoform-specific assembly of holoenzyme complexes can create distinct quaternary structures even though the R(1):C(1) heterodimers are similar in all isoforms. The creation of discrete isoform-specific PKA holoenzyme signaling "foci" paves the way for exploring further biological roles of PKA RIβ and establishes a paradigm for PKA signaling.

Sphingosine kinase interacting protein is an A-kinase anchoring protein specific for type I cAMP-dependent protein kinase

The compartmentalization of kinases and phosphatases plays an important role in the specificity of second-messenger-mediated signaling events. Localization of the cAMP-dependent protein kinase is mediated by interaction of its regulatory subunit (PKA-R) with the versatile family of A-kinase-anchoring proteins (AKAPs). Most AKAPs bind avidly to PKA-RII, while some have dual specificity for both PKA-RI and PKA-RII; however, no mammalian PKA-RI-specific AKAPs have thus far been assigned. This has mainly been attributed to the observation that PKA-RI is more cytosolic than the more heavily compartmentalized PKA-RII. Chemical proteomics screens of the cAMP interactome in mammalian heart tissue recently identified sphingosine kinase type 1-interacting protein (SKIP, SPHKAP) as a putative novel AKAP. Biochemical characterization now shows that SPHKAP can be considered as the first mammalian AKAP that preferentially binds to PKA-RIalpha. Recombinant human SPHKAP functions as an RI-specific AKAP that utilizes the characteristic AKAP amphipathic helix for interaction. Further chemical proteomic screening utilizing differential binding characteristics of specific cAMP resins confirms SPHKAPs endogenous specificity for PKA-RI directly in mammalian heart and spleen tissue. Immunolocalization studies revealed that recombinant SPHKAP is expressed in the cytoplasm, where PKA-RIalpha also mainly resides. Alignment of SPHKAPs' amphipathic helix with peptide models of PKA-RI- or PKA-RII-specific anchoring domains shows that it has largely only PKA-RIalpha characteristics. Being the first mammalian PKA-RI-specific AKAP with cytosolic localization, SPHKAP is a very promising model for studying the function of the less explored cytosolic PKA-RI signaling nodes.

Characterization of a novel tektin member, TEKT5, in mouse sperm

Tektins are important components of flagella. Alterations in the expression of or mutations in mouse tektins are correlated with defective sperm motility, a cause of male infertility. Our proteomic studies of flagellar accessory structures previously identified a novel tektin, TEKT5, whose function is unknown. To understand the role of TEKT5 in mouse sperm, we characterized the expression of the mouse Tekt5 gene and the presence of TEKT5 in spermatogenic cells and spermatozoa. A complete cDNA encoding the Tekt5 transcript was assembled following reverse transcription-polymerase chain reaction (RT-PCR) and 3'-rapid amplification of cDNA ends and predicted that TEKT5 is a 62 730-dalton protein with an unusual, long C-terminus. Tekt5 mRNA was highly expressed during late stages of spermiogenesis. Among examined tissues, Tekt5 mRNA was present only in testis and brain, and quantitative RT-PCR showed that the expression level of mRNA in testis was 6.8-fold higher than that in brain. At the protein level, TEKT5 was present in sperm and was enriched in the accessory structures of flagella. Immunofluorescence confirmed that TEKT5 was localized throughout the sperm tail in flagellar accessory structures. The expression pattern suggests that TEKT5 plays an important role in flagella formation during spermiogenesis as well as being implicated in sperm motility.

Design of proteolytically stable RI-anchoring disruptor peptidomimetics for in vivo studies of anchored type I protein kinase A-mediated signalling

We have reported previously the design of a RIAD (RI-anchoring disruptor) peptide that specifically displaces PKA (protein kinase A) type I from the AKAP (A-kinase-anchoring protein) ezrin, which is present in the immunological synapse of T-cells. This increases immune reactivity by reducing the threshold for activation and may prove a feasible approach for improving immune function in patients with cAMP-mediated T-cell dysfunction. However, the use of RIAD in biological systems is restricted by its susceptibility to enzymatic cleavage and, consequently, its short half-life in presence of the ubiquitous serum peptidases. In the present study, carefully selected non-natural amino acids were employed in the design of RIAD analogues with improved stability. The resulting peptidomimetics demonstrated up to 50-fold increased half-lives in serum compared with RIAD, while maintaining similar or improved specificity and potency with respect to disruption of PKA type I-AKAP interactions.

Selectivity in enrichment of cAMP-dependent protein kinase regulatory subunits type I and type II and their interactors using modified cAMP affinity resins

cAMP regulates cellular functions primarily by activating PKA. The involvement of PKAs in various signaling pathways occurring simultaneously in different cellular compartments necessitates stringent spatial and temporal regulation. This specificity is largely achieved by binding of PKA to protein scaffolds, whereby a distinct group of proteins called A kinase anchoring proteins (AKAPs) play a dominant role. AKAPs are a diverse family of proteins that all bind via a small PKA binding domain to the regulatory subunits of PKA. The binding affinities between PKA and several AKAPs can be different for different isoforms of the regulatory subunits of PKA. Here we employ a combination of affinity chromatography and mass spectrometry-based quantitative proteomics to investigate specificity in PKA-AKAP interactions. Three different immobilized cAMP analogs were used to enrich for PKA and its interacting proteins from several systems; HEK293 and RCC10 cells and rat lung and testis tissues. Stable isotope labeling was used to confidently identify and differentially quantify target proteins and their preferential binding affinity for the three different cAMP analogs. We were able to enrich all four isoforms of the regulatory subunits of PKA and concomitantly identify more than 10 AKAPs. A selective enrichment of the PKA RI isoforms could be achieved; which allowed us to unravel which AKAPs bind preferentially to the RI or RII regulatory domains of PKA. Of the twelve AKAPs detected, seven preferentially bound to RII, whereas the remaining five displayed at least dual specificity with a potential preference for RI. For some of these AKAPs our data provide the first insights into their specificity.

Bicarbonate-induced phosphorylation of p270 protein in mouse sperm by cAMP-dependent protein kinase

Signaling by cAMP-dependent protein kinase (PKA) plays an important role in the regulation of mammalian sperm motility. However, it has not been determined how PKA signaling leads to changes in motility, and specific proteins responsible for these changes have not yet been identified as PKA substrates. Anti-phospho-(Ser/Thr) PKA substrate antibodies detected a sperm protein with a relative molecular weight of 270,000 (p270), which was phosphorylated within 1 min after incubation in a medium supporting capacitation. Phosphorylation of p270 was induced by bicarbonate or a cAMP analog, but was blocked by the PKA inhibitor H-89, indicating that p270 is likely a PKA substrate in sperm. In addition, phosphorylation of p270 was inhibited by stearated peptide st-Ht31, suggesting that p270 is phosphorylated by PKA associated with an A-kinase anchoring protein (AKAP). AKAP4 is the major fibrous sheath protein of mammalian sperm and tethers regulatory subunits of PKA to localize phosphorylation events. Phosphorylation of p270 occurred in sperm lacking AKAP4, suggesting that AKAP4 is not involved directly in the phosphorylation event. Phosphorylated p270 was enriched in fractionated sperm tails and appeared to be present in multiple compartments including a detergent-resistant membrane fraction. PKA phosphorylation of p270 within 1 min of incubation under capacitation conditions suggests that this protein may have an important role in the initial signaling events that lead to the activation and subsequent hyperactivation of sperm motility.

Embracing emerging paradigms of G protein-coupled receptor agonism and signaling to address airway smooth muscle pathobiology in asthma

G protein-coupled receptors (GPCRs) regulate numerous airway cell functions, and signaling events transduced by GPCRs are important in both asthma pathogenesis and therapy. Indeed, most asthma therapies target GPCRs either directly or indirectly. Within recent years, our understating of how GPCRs signal and are regulated has changed significantly as new concepts have emerged and traditional ideas have evolved. In this review, we discuss current concepts regarding constitutive GPCR activity and receptor agonism, functional selectivity, compartmentalized signaling, and GPCR desensitization. We further discuss the relevance of these ideas to asthma and asthma therapy, while emphasizing their potential application to the GPCR signaling in airway smooth muscle that regulates airway patency and thus disease severity.

Regulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) and BIG2 activity via PKA and protein phosphatase 1gamma

Brefeldin A-inhibited guanine nucleotide-exchange proteins (GEPs) BIG1 and BIG2 activate ADP-ribosylation factor (ARF) GTPases, which are required for vesicular trafficking. Both molecules contain one or more sites for binding protein kinase A, i.e., A kinase-anchoring protein (AKAP) sequences. Elevation of cell cAMP caused PKA-catalyzed phosphorylation and nuclear accumulation of BIG1 but not BIG2. We then asked whether BIG1 phosphorylation altered its GEP activity. Incubation of BIG1 or BIG2 with PKA catalytic subunits and ATP resulted in retardation of their electrophoretic migration, consistent with PKA phosphorylation. Okadaic acid inhibits many protein phosphatases, including protein phosphatase 1 (PP1) and PP2A, that can reverse PKA-catalyzed phosphorylation. Incubation of HepG2 cells with okadaic acid caused concentration-dependent accumulation of presumably phosphorylated BIG1 and BIG2 with decreased mobility, which was increased by subsequent incubation in vitro with specific recombinant phosphatases, PP1gamma > PP2A >> PP1alpha. For assays of GEP activity, BIG1 and BIG2 were immunoprecipitated from cells that had been depleted, respectively, of BIG2 and BIG1 by using specific siRNA. GEP activity of each was significantly decreased after incubation with recombinant PKA plus ATP and restored by incubation with PP1gamma. In agreement with a role for PP1gamma in regulation of BIG, endogenous PP1gamma, but not PP1alpha or beta, was immunoprecipitated with BIG1 or BIG2 from microsomal fractions. All observations are consistent with the effects of BIG1 and BIG2 phosphorylation on vesicular trafficking, via alterations in ARF activation and regulatory roles for cAMP, PKA, and PP1gamma in ARF activation by BIG1 and BIG2.

Molecular Basis of AKAP Specificity for PKA Regulatory Subunits

Localization of cyclic AMP (cAMP)-dependent protein kinase (PKA) by A kinase-anchoring proteins (AKAPs) restricts the action of this broad specificity kinase. The high-resolution crystal structures of the docking and dimerization (D/D) domain of the RIIalpha regulatory subunit of PKA both in the apo state and in complex with the high-affinity anchoring peptide AKAP-IS explain the molecular basis for AKAP-regulatory subunit recognition. AKAP-IS folds into an amphipathic alpha helix that engages an essentially preformed shallow groove on the surface of the RII dimer D/D domains. Conserved AKAP aliphatic residues dominate interactions to RII at the predominantly hydrophobic interface, whereas polar residues are important in conferring R subunit isoform specificity. Using a peptide screening approach, we have developed SuperAKAP-IS, a peptide that is 10,000-fold more selective for the RII isoform relative to RI and can be used to assess the impact of PKA isoform-selective anchoring on cAMP-responsive events inside cells.

Distinct interaction modes of an AKAP bound to two regulatory subunit isoforms of protein kinase A revealed by amide hydrogen/deuterium exchange

The structure of an AKAP docked to the dimerization/docking (D/D) domain of the type II (RIIalpha) isoform of protein kinase A (PKA) has been well characterized, but there currently is no detailed structural information of an AKAP docked to the type I (RIalpha) isoform. Dual-specific AKAP2 (D-AKAP2) binds in the nanomolar range to both isoforms and provided us with an opportunity to characterize the isoform-selective nature of AKAP binding using a common docked ligand. Hydrogen/deuterium (H/D) exchange combined with mass spectrometry (DXMS) was used to probe backbone structural changes of an alpha-helical A-kinase binding (AKB) motif from D-AKAP2 docked to both RIalpha and RIIalpha D/D domains. The region of protection upon complex formation and the magnitude of protection from H/D exchange were determined for both interacting partners in each complex. The backbone of the AKB ligand was more protected when bound to RIalpha compared to RIIalpha, suggesting an increased helical stabilization of the docked AKB ligand. This combined with a broader region of backbone protection induced by the AKAP on the docking surface of RIalpha indicated that there were more binding constraints for the AKB ligand when bound to RIalpha. This was in contrast to RIIalpha, which has a preformed, localized binding surface. These distinct modes of AKAP binding may contribute to the more discriminating nature of the RIalpha AKAP-docking surface. DXMS provides valuable structural information for understanding binding specificity in the absence of a high-resolution structure, and can readily be applied to other protein-ligand and protein-protein interactions.

Changes in intracellular distribution and activity of protein phosphatase PP1gamma2 and its regulating proteins in spermatozoa lacking AKAP4

The second messenger cAMP mediates its intracellular effects in spermatozoa through cAMP-dependent kinase (PKA, formally known as PRKACA). The intracellular organization of PKA in spermatozoa is controlled through its association with A-kinase-anchoring proteins (AKAPs). AKAP4 (A kinase [PRKA] anchor protein 4; also called fibrous sheath component 1 or AKAP 82) is sperm specific and the major fibrous sheath protein of the principal piece of the sperm flagellum. Presumably, AKAP4 recruits PKA to the fibrous sheath and facilitates local phosphorylation to regulate flagellar function. It is also proposed to act as a scaffolding protein for signaling proteins and proteins involved in metabolism. Akap4 gene knockout mice are infertile due to the lack of sperm motility. The fibrous sheath is disrupted in spermatozoa from mutant mice. In this article, we used Akap4 gene knockout mice to study the effect of fibrous sheath disruption on the presence, subcellular distribution, and/or activity changes of PKA catalytic and regulatory subunits, sperm flagellum proteins PP1gamma2 (protein phosphatase 1, catalytic subunit, gamma isoform, formally known as PPP1CC), GSK-3 (glycogen synthase kinase-3), SP17 (sperm autoantigenic protein 17, formally known as SPA17), and other signaling proteins. There were no changes in the presence and subcellular distribution for PP1gamma2, GSK-3, hsp90 (heat shock protein 1, alpha, formally known as HSPCA), sds22 (protein phosphatase 1, regulatory [inhibitor] subunit 7, formally known as PPP1R7), 14-3-3 protein (tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein), and PKB (thymoma viral proto-oncogene, also known as AKT) in mutant mice. However, the subcellular distributions for PKA catalytic subunit and regulatory subunits, PI 3-kinase (phosphatidylinositol 3-kinase), and SP17 were disrupted in mutant mice. Furthermore, there was a significant change in the activity and phosphorylation of PP1gamma2 in mutant compared with wild-type spermatozoa. These studies have identified potentially significant new roles for the fibrous sheath in regulating the activity and function of key signaling enzymes.

Related protein-protein interaction modules present drastically different surface topographies despite a conserved helical platform

The subcellular localization of cAMP-dependent protein kinase (PKA) occurs through interaction with A-Kinase Anchoring Proteins (AKAPs). AKAPs bind to the PKA regulatory subunit dimer of both type Ialpha and type IIalpha (RIalpha and RIIalpha). RIalpha and RIIalpha display characteristic localization within different cell types, which is maintained by interaction of AKAPs with the N-terminal dimerization and docking domain (D/D) of the respective regulatory subunit. Previously, we reported the solution structure of RIIa D/D module, both free and bound to AKAPs. We have now solved the solution structure of the dimerization and docking domain of the type Ialpha regulatory dimer subunit (RIalpha D/D). RIalpha D/D is a compact docking module, with unusual interchain disulfide bonds that help maintain the AKAP interaction surface. In contrast to the shallow hydrophobic groove for AKAP binding across the surface of the RIIalpha D/D dimeric interface, the RIalpha D/D module presents a deep cleft for proposed AKAP binding. RIalpha and RIIalpha D/D interaction modules present drastically differing dimeric topographies, despite a conserved X-type four-helix bundle structure.

AKAP7gamma is a nuclear RI-binding AKAP

Spatial regulation of protein kinase A (PKA) is accomplished by its sequestration via A-kinase anchor proteins (AKAPs). PKA activity is critical for mammalian oocyte development, suggesting that PKA must be appropriately positioned in these large cells. A screen for AKAPs in oocytes identified AKAP7gamma, an AKAP originally found in pancreas. Yeast two-hybrid analysis and co-immunoprecipitation studies showed that AKAP7gamma bound the type I PKA regulatory subunit (RI) and that the RI-binding domain overlapped the previously identified type II PKA regulatory subunit (RII) binding domain. Overexpressed AKAP7gamma localized to the nuclei of HEK 293 cells via a nuclear localization signal. In addition, endogenous AKAP7gamma protein was found in both the nucleus and cytoplasm of oocytes. This work identifies AKAP7gamma as the first nuclear AKAP to bind RI and suggests that AKAP7gamma may be responsible for positioning PKA via RI and/or RII to regulate PKA-mediated gene transcription in both somatic cells and oocytes.

A-kinase anchoring protein 4 binding proteins in the fibrous sheath of the sperm flagellum

The fibrous sheath is a unique cytoskeletal structure located in the principal piece of the sperm flagellum and is constructed of two longitudinal columns connected by closely spaced circumferential ribs. Cyclic AMP-dependent protein kinases are secured within specific cytoplasmic domains by A-kinase anchoring proteins (AKAPs), and the most abundant protein in the fibrous sheath is AKAP4. Several other fibrous sheath proteins have been identified, but how the fibrous sheath assembles is not understood. Yeast two-hybrid assays and deletion mutagenesis were used to identify AKAP4-binding proteins and to map the binding regions on AKAP4 and on the proteins identified. We found that AKAP4 binds AKAP3 and two novel spermatogenic cell-specific proteins, Fibrous Sheath Interacting Proteins 1 and 2 (FSIP1, FSIP2). Transcription of Akap4, Akap3, and Fsip1 begins in early spermatid development, whereas transcription of Fsip2 begins in late spermatocyte development. AKAP3 is synthesized in round spermatids and incorporated into the fibrous sheath concurrently with formation of the rib precursors. However, AKAP4 is synthesized and incorporated into the nascent fibrous sheath late in spermatid development. The AKAP4 precursor is processed in the flagellum and only the mature form of AKAP4 appears to bind AKAP3. These results suggest that AKAP3 is involved in organizing the basic structure of the fibrous sheath, whereas AKAP4 has a major role in completing fibrous sheath assembly.

Bioinformatic design of A-kinase anchoring protein-in silico: a potent and selective peptide antagonist of type II protein kinase A anchoring

Compartmentalization of the cAMP-dependent protein kinase (PKA) is coordinated through association with A-kinase anchoring proteins (AKAPs). A defining characteristic of most AKAPs is a 14- to 18-aa sequence that binds to the regulatory subunits (RI or RII) of the kinase. Cellular delivery of peptides to these regions disrupts PKA anchoring and has been used to delineate a physiological role for AKAPs in the facilitation of certain cAMP-responsive events. Here, we describe a bioinformatic approach that yields an RII-selective peptide, called AKAP-in silico (AKAP-IS), that binds RII with a K(d) of 0.4 nM and binds RI with a K(d) of 277 nM. AKAP-IS associates with the type II PKA holoenzyme inside cells and displaces the kinase from natural anchoring sites. Electrophysiological recordings indicate that perfusion of AKAP-IS evokes a more rapid and complete attenuation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor currents than previously described anchoring inhibitor peptides. Thus, computer-based and peptide array screening approaches have generated a reagent that binds PKA with higher affinity than previously described AKAPs.

Amino acid variant in the kinase binding domain of dual-specific A kinase-anchoring protein 2: a disease susceptibility polymorphism

The focus of human genetics in recent years has shifted toward identifying genes that are involved in the development of common diseases such as cancer, diabetes, cardiovascular diseases, and Alzheimer's disease. Because many complex diseases are late-onset, the frequencies of disease susceptibility alleles are expected to decrease in the healthy elderly individuals of the population at large because of their contribution to disease morbidity andor mortality. To test this assumption, we compared allele frequencies of 6,500 single-nucleotide polymorphisms (SNPs) located in approximately 5,000 genes between DNA pools of age-stratified healthy, European-American individuals. A SNP that results in an amino acid change from Ile to Val in the dual-specific A kinase-anchoring protein 2 (d-AKAP2) gene, showed the strongest correlation with age. Subsequent analysis of an independent sample indicated that the Val variant was associated with a statistically significant decrease in the length of the electrocardiogram PR interval. The IleVal SNP is located in the A-kinase-binding domain. An in vitro binding assay revealed that the Ile variant bound approximately 3-fold weaker to the protein kinase A (PKA)-RIalpha isoform than the Val variant. This decreased affinity resulted in alterations in the subcellular distribution of the recombinantly expressed PKA-RIalpha isoform. Our study suggests that alterations in PKA-RIalpha subcellular localization caused by variation in d-AKAP2 may have a negative health prognosis in the aging population, which may be related to cardiac dysfunction. Age-stratified samples appear to be useful for screening SNPs to identify functional gene variants that have an impact on health.

Protein kinase A-anchoring (AKAP) domains in brefeldin A-inhibited guanine nucleotide-exchange protein 2 (BIG2)

Like other guanine nucleotide-exchange proteins (GEPs) that activate ADP-ribosylation factor (ARF) GTPases, brefeldin A-inhibited GEP2, BIG2, contains an approximately 200-aa Sec7 domain that is responsible for this catalytic activity and its inhibition by brefeldin A. The Sec7 domain is located near the center of the molecule and serves to accelerate replacement of GDP bound to ARF with GTP. To explore possible functions of the N-terminal region of BIG2 (1-832), we used three coding-region constructs as bait to screen a human heart cDNA library in a yeast two-hybrid system, retrieving two unique clones that encode a type I protein kinase A (PKA) regulatory subunit, RI alpha. Coimmunoprecipitation experiments confirmed interaction of in vitro translated BIG2 and RI alpha, as well as of the endogenous proteins in cytosol of cultured HepG2 cells. Using 28 deletion mutants, we found three regions of BIG2 that interacted with R subunits of PKA. Residues 27-48 (domain A) interacted with RI alpha and RI beta, 284-301 (domain B) interacted with RII alpha and RII beta, and 517-538 (domain C) interacted with RI alpha, RII alpha, and RII beta. Sequence analysis and helical wheel projection of amino acids in the three domains revealed potential amphipathic wheel structures characteristic for binding of PKA R subunits. Western blot analysis of subcellular fractions demonstrated translocation of BIG2 (and BIG1) from cytosol to the Golgi and other membrane structures after incubation of cells with 8-Br-cAMP or forskolin. All findings are consistent with a role for BIG2 as an A kinase-anchoring protein (or AKAP) that could coordinate cAMP and ARF regulatory pathways.

Essential role of A-kinase anchor protein 121 for cAMP signaling to mitochondria

A-Kinase anchor proteins (AKAPs) immobilize and concentrate protein kinase A (PKA) isoforms at specific subcellular compartments. Intracellular targeting of PKA holoenzyme elicits rapid and efficient phosphorylation of target proteins, thereby increasing sensitivity of downstream effectors to cAMP action. AKAP121 targets PKA to the cytoplasmic surface of mitochondria. Here we show that conditional expression of AKAP121 in PC12 cells selectively enhances cAMP.PKA signaling to mitochondria. AKAP121 induction stimulates PKA-dependent phosphorylation of the proapoptotic protein BAD at Ser(155), inhibits release of cytochrome c from mitochondria, and protects cells from apoptosis. An AKAP121 derivative mutant that localizes on mitochondria but does not bind PKA down-regulates PKA signaling to the mitochondria and promotes apoptosis. These findings indicate that PKA anchored by AKAP121 transduces cAMP signals to the mitochondria, and it may play an important role in mitochondrial physiology.

Characterization of an A-kinase anchoring protein in human ciliary axonemes

Although protein kinase A (PKA) activation is known to increase ciliary beat frequency in humans the molecular mechanisms involved are unknown. We demonstrate that PKA is associated with ciliary axonemes where it specifically phosphorylates a 23-kDa protein. Because PKA is often localized to subcellular compartments in proximity to its substrate(s) via interactions with A-kinase-anchoring proteins (AKAPs), we investigated whether an AKAP was also associated with ciliary axonemes. This study has identified a novel 28 kDa AKAP (AKAP28)that is highly enriched in airway axonemes. The mRNA for AKAP28 is up-regulated as primary airway cells differentiate and is specifically expressed in tissues containing cilia and/or flagella. Additionally, both Western blot and immunostaining data show that AKAP28 is enriched in airway cilia. These data demonstrate that we have identified the first human axonemal AKAP, a protein that likely plays a role in the signaling necessary for efficient modulation of ciliary beat frequency.

A-kinase anchor proteins as potential regulators of protein kinase A function in oocytes

In the mammalian oocyte, the cAMP-dependent protein kinase (PKA) has critical functions in the maintenance of meiotic arrest and oocyte maturation. Because PKA is spatially regulated, its localization was examined in developing oocytes. Both regulatory subunits (RI and RII) and the catalytic subunit (C) of PKA were found in oocytes and metaphase II-arrested eggs. In the oocyte, RI and C were predominantly localized in the cortical region, while RII showed a punctate distribution within the cytoplasm. After maturation to metaphase II, RI remained in the cortex and was also localized to the meiotic spindle, while RII was found adjacent to the spindle. C was diffuse within the cytoplasm of the egg but was enriched in the cytoplasm surrounding the metaphase spindle, much like RII. The polarized localization and redistribution of RI, RII, and C suggested that PKA might be tethered by A-kinase anchor proteins (AKAPs), proteins that tether PKA close to its physiological substrates. An AKAP, AKAP140, was identified that was developmentally regulated and phosphorylated in oocytes and eggs. AKAP140 was shown to be a dual-specific AKAP, having the ability to bind both RI and RII. By compartmentalizing PKA, AKAP140 and/or other AKAPs could spatially regulate PKA activity during oocyte development.

Targeted disruption of the Akap4 gene causes defects in sperm flagellum and motility

A-kinase anchoring proteins (AKAPs) tether cyclic AMP-dependent protein kinases and thereby localize phosphorylation of target proteins and initiation of signal-transduction processes triggered by cyclic AMP. AKAPs can also be scaffolds for kinases and phosphatases and form macromolecular complexes with other proteins involved in signal transduction. Akap4 is transcribed only in the postmeiotic phase of spermatogenesis and encodes the most abundant protein in the fibrous sheath, a novel cytoskeletal structure present in the principal piece of the sperm flagellum. Previous studies indicated that cyclic AMP-dependent signaling processes are important in the regulation of sperm motility, and gene targeting was used here to test the hypothesis that AKAP4 is a scaffold for protein complexes involved in regulating flagellar function. Sperm numbers were not reduced in male mice lacking AKAP4, but sperm failed to show progressive motility and male mice were infertile. The fibrous sheath anlagen formed, but the definitive fibrous sheath did not develop, the flagellum was shortened, and proteins usually associated with the fibrous sheath were absent or substantially reduced in amount. However, the other cytoskeletal components of the flagellum were present and appeared fully developed. We conclude that AKAP4 is a scaffold protein required for the organization and integrity of the fibrous sheath and that effective sperm motility is lost in the absence of AKAP4 because signal transduction and glycolytic enzymes fail to become associated with the fibrous sheath.

Increased basal cAMP-dependent protein kinase activity inhibits the formation of mesoderm-derived structures in the developing mouse embryo

A targeted disruption of the RIalpha isoform of protein kinase A (PKA) was created by using homologous recombination in embryonic stem cells. Unlike the other regulatory and catalytic subunits of PKA, RIalpha is the only isoform that is essential for early embryonic development. RIalpha homozygous mutant embryos fail to develop a functional heart tube at E8.5 and are resorbed at approximately E10.5. Mutant embryos show significant growth retardation and developmental delay compared with wild type littermates from E7.5 to E10.5. The anterior-posterior axis of RIalpha mutants is well developed, with a prominent head structure but a reduced trunk. PKA activity measurements reveal an increased basal PKA activity in these embryos. Brachyury mRNA expression in the primitive streak of RIalpha mutants is significantly reduced, consistent with later deficits in axial, paraxial, and lateral plate mesodermal derivatives. This defect in the production and migration of mesoderm can be completely rescued by crossing RIalpha mutants to mice carrying a targeted disruption in the Calpha catalytic subunit, demonstrating that unregulated PKA activity rather than a specific loss of RIalpha is responsible for the phenotype. Primary embryonic fibroblasts from RIalpha mutant embryos display an abnormal cytoskeleton and an altered ability to migrate in cell culture. Our results demonstrate that unregulated PKA activity negatively affects growth factor-mediated mesoderm formation during early mouse development.

A-kinase anchor protein 84/121 are targeted to mitochondria and mitotic spindles by overlapping amino-terminal motifs

A-kinase anchor proteins (AKAPs) assemble multi-enzyme signaling complexes in proximity to substrate/effector proteins, thus directing and amplifying membrane-generated signals. S-AKAP84 and AKAP121 are alternative splicing products with identical NH(2) termini. These AKAPs bind and target protein kinase A (PKA) to the outer mitochondrial membrane. Tubulin was identified as a binding partner of S-AKAP84 in a yeast two-hybrid screen. Immunoprecipitation and co-sedimentation experiments in rat testis extracts confirmed the interaction between microtubules and S-AKAP84. In situ immunostaining of testicular germ cells (GC2) shows that AKAP121 concentrates on mitochondria in interphase and on mitotic spindles during M phase. Purified tubulin binds directly to S-AKAP84 but not to a deletion mutant lacking the mitochondrial targeting domain (MT) at residues 1-30. The MT is predicted to form a highly hydrophobic alpha-helical wheel that might also mediate interaction with tubulin. Disruption of the wheel by site-directed mutagenesis abolished tubulin binding and reduced mitochondrial attachment of an MT-GFP fusion protein. Some MT mutants retain tubulin binding but do not localize to mitochondria. Thus, the tubulin-binding motif lies within the mitochondrial attachment motif. Our findings indicate that S-AKAP84/AKAP121 use overlapping targeting motifs to localize signaling enzymes to mitochondrial and cytoskeletal compartments.

Tumor necrosis factor receptor 1 is an ATPase regulated by silencer of death domain

Self-aggregation of tumor necrosis factor receptor type 1 (TNFR1) induces spontaneous downstream signaling and results in cell death. It has been suggested that silencer of death domain (SODD) binds TNFR1 monomers to prevent self-aggregation. We found that SODD binds through its BAG domain to the ATPase domain of Hsp70. We also determined that SODD binds through its BAG domain to TNFR1. ATP, but not nonhydrolyzable ATP-gamma S, regulates the SODD binding by Hsp70 or TNFR1. ATP binding by TNFR1 was abolished when a point mutation was introduced into a phosphate-binding loop motif characteristic of ATP-binding proteins, suggesting that TNFR1 functions as an ATPase. Furthermore, TNFR1 was present in aggregates in ATP-depleted cells and SODD disassembled aggregates in vitro only in the presence of ATP. These data suggest that SODD functions as a cofactor analogous to the nucleotide exchange factor BAG-1, which modulates the ATPase cycle of Hsp70 proteins. We propose a new model in which a nucleotide-dependent conformational change in TNFR1 has a key role in regulating TNF signaling.

Identification, localization, and function in steroidogenesis of PAP7: a peripheral-type benzodiazepine receptor- and PKA (RIalpha)-associated protein

Peptide hormones and cAMP acutely stimulate steroid biosynthesis by accelerating the transport of cholesterol into the mitochondria. The peripheral-type benzodiazepine receptor (PBR) has been shown to be an indispensable element of the cholesterol transport machinery. Using the yeast two-hybrid system and PBR as bait, we identified a protein that interacts with PBR, the PBR-associated protein PAP7. Using the regulatory subunit RIalpha of PKA as bait, we also isolated PAP7. Glutathione-S-transferase -PAP7 interacted with both the mitochondrial PBR and cytosolic PKA-RIalpha in MA-10 Leydig cells. PAP7 is a novel 52-kDa protein present in mouse, rat, and human tissues, and it has a major 3-kb mRNA transcript in all tissues examined. Immunohistochemical and in situ hybridization studies indicated that PAP7 is highly expressed in the gonads, adrenal, hippocampus, and distinct brain neuronal and glial populations. Overexpression of the full length PAP7 increased the hCG-induced steroid production. However, overexpression of a partial PAP7, which includes the PBR- and PKA-RIalpha-binding domains, inhibited the hormone-stimulated cholesterol transport and steroid synthesis. Treatment of MA-10 cells with oligonucleotides antisense to PAP7 also inhibited the hCG-stimulated steroid formation, suggesting that PAP7 is a functional element of the hormone-induced signal transduction cascade leading to steroidogenesis. PAP7 may function by targeting the PKA isoenzyme to organelles rich in PBR, i.e. mitochondria, where phosphorylation of specific protein substrates may induce the reorganization of PBR topography and function.

Cyclic AMP-dependent protein kinase isoenzymes in human myeloid leukemia (HL60) and breast tumor (MCF-7) cells

Combinations of retinoic acid (RA) and cAMP mediate many biological responses in a large variety of cell types. While the basis for the apparent synergistic effects of RA and cAMP are not clearly defined, it is likely that activation of PKA by cAMP is involved. However, literature reports concerning the identity of PKA isoforms in HL60 and MCF-7 cells are conflicting. The purpose of the present investigation is to identify PKA isoforms in HL60 and MCF-7 cells. Utilization of high-performance anion-exchange liquid chromatography, immunoblotting, and 8-azido-cAMP photoaffinity binding resulted in the finding that HL60 cells contain PKA types I alpha and II alpha, while MCF-7 cells contain PKA types I alpha, II alpha, and II beta. PKA type I alpha in both HL60 and MCF-7 cells eluted from columns as two well-separated peaks. One peak eluted at a low salt concentration in agreement with previous reports. The second HL60 PKA type I alpha peak eluted at a salt concentration intermediate between that eluting the first peak and that eluting PKA type II alpha and contained approximately 62% of the total RI alpha protein. However, the second MCF-7 PKA type I alpha peak contained approximately 66% of the total RI alpha protein and co-eluted with PKA types II alpha and II beta. This "contamination" of PKA type II fractions with PKA type I has led, in some cases, to interpretations that may need reevaluation.

Role and identification of protein kinase A anchoring proteins in vasopressin-mediated aquaporin-2 translocation

The antidiuretic hormone arginine vasopressin (AVP) regulates water reabsorption in renal principal cells by inducing a cAMP/protein kinase A-dependent translocation of water channels [aquaporin-2 (AQP2)] from intracellular vesicles into the apical cell membranes. Using primary cultured rat inner medullary collecting duct (IMCD) cells, it has been shown that AQP2 translocation in response to AVP stimulation occurs only if protein kinase A (PKA) is anchored to PKA anchoring proteins (AKAPs), which are present in various subcellular compartments. The identity of the AKAPs involved has not yet been elucidated. One potential candidate is a new splice variant of AKAP18, namely AKAP18 delta.

Binding of PKA-RIIalpha to the Adenovirus E1A12S oncoprotein correlates with its nuclear translocation and an increase in PKA-dependent promoter activity

The adenovirus type 12 (Ad12) E1A12S oncoprotein utilizes the cAMP/protein kinase A (PKA) signal transduction pathway to activate expression of the viral E2 gene, the products of which are essential for viral replication. A central unsolved question is, however, whether E1A12S interacts directly with PKA in the process of promoter activation. We show here that E1A12S binds to the regulatory subunits (R) of PKA in vitro and in vivo. Interaction depends on the N-terminus and the conserved region 1 (CR1) of E1A12S. Both domains are also essential for the activation of viral E2 gene expression. Infection of cells with Ad12 leads to the cellular redistribution of RIIalpha from the cytoplasm into the nucleus. Furthermore, RIIalpha is also located in the nucleus of cells transformed by E1 of Ad12 and transient expression of E1A12S leads to the redistribution of RIIalpha into the nucleus in a N-terminus- and CR1-dependent manner. Cotransfection of E1A12S with RIIalpha results in strong activation of the E2 promoter. Based on these results we conclude that E1A12S functions as a viral A-kinase anchoring protein redistributing RIIalpha from the cytoplasm into the nucleus where it is involved in E1A12S-mediated activation of the E2 promoter.

Identification of sperm-specific proteins that interact with A-kinase anchoring proteins in a manner similar to the type II regulatory subunit of PKA

The cAMP-dependent protein kinase (PKA) is targeted to specific subcellular compartments through its interaction with A-kinase anchoring proteins (AKAPs). AKAPs contain an amphipathic helix domain that binds to the type II regulatory subunit of PKA (RII). Synthetic peptides containing this amphipathic helix domain bind to RII with high affinity and competitively inhibit the binding of PKA with AKAPs. Addition of these anchoring inhibitor peptides to spermatozoa inhibits motility (Vijayaraghavan, S., Goueli, S. A., Davey, M. P., and Carr, D. W. (1997) J. Biol. Chem. 272, 4747-4752). However, inhibition of the PKA catalytic activity does not mimic these peptides, suggesting that the peptides are disrupting the interaction of AKAP(s) with proteins other than PKA. Using the yeast two-hybrid system, we have now identified two sperm-specific human proteins that interact with the amphipathic helix region of AKAP110. These proteins, ropporin (a protein previously shown to interact with the Rho signaling pathway) and AKAP-associated sperm protein, are 39% identical to each other and share a strong sequence similarity with the conserved domain on the N terminus of RII that is involved in dimerization and AKAP binding. Mutation of conserved residues in ropporin or RII prevents binding to AKAP110. These data suggest that sperm contains several proteins that bind to AKAPs in a manner similar to RII and imply that AKAPs may have additional and perhaps unique functions in spermatozoa.

The biological functions of A-kinase anchor proteins

cAMP-dependent protein kinase is targeted to discrete subcellular locations by a family of specific anchor proteins (A-kinase anchor proteins, AKAPs). Localization recruits protein kinase A (PKA) holoenzyme close to its substrate/effector proteins, directing and amplifying the biological effects of cAMP signaling.AKAPs include two conserved structural modules: (i) a targeting domain that serves as a scaffold and membrane anchor; and (ii) a tethering domain that interacts with PKA regulatory subunits. Alternative splicing can shuffle targeting and tethering domains to generate a variety of AKAPs with different targeting specificity. Although AKAPs have been identified on the basis of their interaction with PKA, they also bind other signaling molecules, mainly phosphatases and kinases, that regulate AKAP targeting and activate other signal transduction pathways. We suggest that AKAP forms a "transduceosome" by acting as an autonomous multivalent scaffold that assembles and integrates signals derived from multiple pathways. The transduceosome amplifies cAMP and other signals locally and, by stabilizing and reducing the basal activity of PKA, it also exerts long-distance effects. The AKAP transduceosome thus optimizes the amplitude and the signal/noise ratio of cAMP-PKA stimuli travelling from the membrane to the nucleus and other subcellular compartments.