A-kinase anchoring proteins take shape

Abstract

A-kinase anchoring proteins (AKAPs) are signaling scaffolds that contribute to various aspects of cAMP signaling. They do this by tethering protein kinase-A to specific subcellular sites, thereby focusing its activity toward relevant substrates. Recently the structural basis for these protein-protein interactions has been elucidated by x-ray crystallography. Recent reports have identified AKAPs that bind to adenylyl cyclases to regulate cAMP synthesis and that sequester phosphodiesterases to break down this second messenger locally. Another emerging aspect of AKAP function is their role in integrating cAMP signaling with other signaling pathways. For example, molecular and genetic approaches have been used to show that the neuronal anchoring protein WAVE1 integrates signaling from PKA and Cdk5 to regulate actin polymerization and cytoskeletal events.

Figure 1

Structure of the PKA/AKAP binding interface. (a) Schematic of an AKAP PKA-anchoring domain (yellow) bound to the D/D domain (residues 1–44) of RII. In the structures shown below, the helical backbones of AKAPis and DAKAP2 are depicted in yellow and residues that form the binding interface between RII and the AKAP (red) are shown in space-filling. The primary sequences of the AKAP peptides are included with interfacial residues highlighted in red. (b) Close-up of the AKAPis helix bound to the hydrophobic groove of RII. AKAPis residues that form polar interactions with RII are shown in ball and stick (yellow). (c) Close-up of the DAKAP2 PKA-anchoring domain bound to the hydrophobic groove of RII. The potential steric discriminator that contributes to the PKA-subtype specificity of AKAPs is indicated. Comparison of the two structures shows that, although they are quite similar, the RII/AKAPis binding interface comprises a greater number of residues and there are differences in the registry of the helical side-chains involved at each interface.

Figure 2

Compartmentalized cAMP signaling. Schematic depicting the role of various AKAPs in regulating the synthesis and degradation of cAMP. These AKAP-mediated processes contribute to the generation of local cAMP gradients in the cell and ultimately the spatiotemporal dynamics of the cAMP effectors PKA and EPACs.

Figure 3

AKAP-mediated signal integration. (a) Schematic of the role AKAP–Lbc plays in integrating cAMP signaling with both PKD and Rho pathways. Activation of PKD (blue numbers): (1) AKAP-Lbc colocalizes PKD near PKC, its upstream activator; (2) PKC phosphorylates PKD; (3) Dissociation of active PKD from the AKAP platform is promoted by PKA phosphorylation of AKAP-Lbc. Rho inhibition (purple numbers): (1) PKA phosphorylation of AKAP-Lbc generates a 14-3-3 binding site; (2) binding of 14-3-3 downregulates the Rho-GEF activity of AKAP-Lbc and (3) inhibits Rho. (b) Schematic of the role WAVE1 plays in coordinating the bi-directional regulation of the Arp2/3 complex by Cdk5 and PKA. Cdk5 phosphorylation of WAVE1 leads to inhibition of Arp2/3. This is opposed by the activity of PKA.