AKAP signaling complexes: getting to the heart of the matter.

Subcellular compartmentalization of protein kinases and phosphatases through their interaction with A-kinase anchoring proteins (AKAPs) provides a mechanism to control signal transduction events at specific sites within the cell. Recent findings suggest that these anchoring proteins dynamically assemble different cAMP effectors to control the cellular actions of cAMP spatially and temporally. In the heart, signaling events such as the onset of cardiac hypertrophy are influenced by muscle-specific mAKAP signaling complexes that target protein kinase A (PKA), the cAMP-responsive guanine-nucleotide exchange factor EPAC and cAMP-selective phosphodiesterase 4 (PDE4). Mediation of signaling events by AKAPs might also have a role in the control of lipolysis in adipocytes, where insulin treatment reduces the association of AKAPs with G-protein-coupled receptors. These are only two examples of how AKAPs contribute to specificity in cAMP signaling. This review will explore recent development that illustrates the role of multiprotein complexes in the regulation of cAMP signaling.

High-affinity AKAP7delta-protein kinase A interaction yields novel protein kinase A-anchoring disruptor peptides.

PKA (protein kinase A) is tethered to subcellular compartments by direct interaction of its regulatory subunits (RI or RII) with AKAPs (A kinase-anchoring proteins). AKAPs preferentially bind RII subunits via their RII-binding domains. RII-binding domains form structurally conserved amphipathic helices with unrelated sequences. Their binding affinities for RII subunits differ greatly within the AKAP family. Amongst the AKAPs that bind RIIalpha subunits with high affinity is AKAP7delta [AKAP18delta; K(d) (equilibrium dissociation constant) value of 31 nM]. An N-terminally truncated AKAP7delta mutant binds RIIalpha subunits with higher affinity than the full-length protein presumably due to loss of an inhibitory region [Henn, Edemir, Stefan, Wiesner, Lorenz, Theilig, Schmidtt, Vossebein, Tamma, Beyermann et al. (2004) J. Biol. Chem. 279, 26654-26665]. In the present study, we demonstrate that peptides (25 amino acid residues) derived from the RII-binding domain of AKAP7delta bind RIIalpha subunits with higher affinity (K(d)=0.4+/-0.3 nM) than either full-length or N-terminally truncated AKAP7delta, or peptides derived from other RII binding domains. The AKAP7delta-derived peptides and stearate-coupled membrane-permeable mutants effectively disrupt AKAP-RII subunit interactions in vitro and in cell-based assays. Thus they are valuable novel tools for studying anchored PKA signalling. Molecular modelling indicated that the high affinity binding of the amphipathic helix, which forms the RII-binding domain of AKAP7delta, with RII subunits involves both the hydrophobic and the hydrophilic faces of the helix. Alanine scanning (25 amino acid peptides, SPOT technology, combined with RII overlay assays) of the RII binding domain revealed that hydrophobic amino acid residues form the backbone of the interaction and that hydrogen bond- and salt-bridge-forming amino acid residues increase the affinity of the interaction.

Rapid establishment of the European Bank for induced Pluripotent Stem Cells (EBiSC) - the Hot Start experience.

A fast track "Hot Start" process was implemented to launch the European Bank for Induced Pluripotent Stem Cells (EBiSC) to provide early release of a range of established control and disease linked human induced pluripotent stem cell (hiPSC) lines. Established practice amongst consortium members was surveyed to arrive at harmonised and publically accessible Standard Operations Procedures (SOPs) for tissue procurement, bio-sample tracking, iPSC expansion, cryopreservation, qualification and distribution to the research community. These were implemented to create a quality managed foundational collection of lines and associated data made available for distribution. Here we report on the successful outcome of this experience and work flow for banking and facilitating access to an otherwise disparate European resource, with lessons to benefit the international research community. ETOC: The report focuses on the EBiSC experience of rapidly establishing an operational capacity to procure, bank and distribute a foundational collection of established hiPSC lines. It validates the feasibility and defines the challenges of harnessing and integrating the capability and productivity of centres across Europe using commonly available resources currently in the field.

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.

Interfacial kinetic analysis of the tumour suppressor phosphatase, PTEN: evidence for activation by anionic phospholipids.

We investigated the kinetic behaviour and substrate specificity of PTEN (phosphatase and tensin homologue deleted on chromosome 10) using unilamellar vesicles containing substrate lipids in a background of phosphatidylcholine. PTEN displays the characteristics expected of an interfacial enzyme, since the rate of enzyme activity is dependent on the surface concentration of the substrate lipids used (mol fraction), as well as the bulk concentration. Surface-dilution analysis revealed the catalytic efficiency of PTEN for PtdIns(3,4,5) P (3) to be 200-fold greater than for either PtdIns(3,4) P (2) or PtdIns(3,5) P (2), and 1000-fold greater than for PtdIns3 P. The interfacial K (m) value of PTEN for PtdIns(3,4,5) P (3) was very low, reflecting the small proportions of this lipid that are present in cellular membranes. The catalytic-centre activity ( k (cat)) for PtdIns(3,4,5) P (3) was at least 200-fold greater than that for the water-soluble substrate Ins(1,3,4,5) P (4). The preference for lipid substrates may result from an interfacial activation of the enzyme, rather than processive catalysis of vesicular substrates. Moreover, both PtdIns(4,5) P (2) and univalent salts stimulated the activity of PTEN for PtdIns(3,4,5) P (3), but profoundly inhibited activity against Ins(1,3,4,5) P (4). The stimulatory effect of PtdIns(4,5) P (2) was greater in magnitude and more potent in comparison with other anionic phospholipid species. A mutation in the lipid-binding C2 domain (M-CBR3) that is biologically inactive did not alter overall catalytic efficiency in this model, but decreased the efficiency of the interfacial binding step, demonstrating its importance in the catalytic mechanism of PTEN.

AKAP-Lbc nucleates a protein kinase D activation scaffold.

The transmission of cellular signals often proceeds through multiprotein complexes where enzymes are positioned in proximity to their upstream activators and downstream substrates. In this report we demonstrate that the A-kinase anchoring protein AKAP-Lbc assembles an activation complex for the lipid-dependent enzyme protein kinase D (PKD). Using a combination of biochemical, enzymatic, and immunofluorescence techniques, we show that the anchoring protein contributes to PKD activation in two ways: it recruits an upstream kinase PKCeta and coordinates PKA phosphorylation events that release activated protein kinase D. Thus, AKAP-Lbc synchronizes PKA and PKC activities in a manner that leads to the activation of a third kinase. This configuration illustrates the utility of kinase anchoring as a mechanism to constrain the action of broad-spectrum enzymes.