The biological functions of A-kinase anchor proteins

doi:10.1006/jmbi.2001.4585

Journal of Molecular Biology

Volume 308, Issue 2, 27 April 2001, Pages 99-114

https://doi.org/10.1006/jmbi.2001.4585 Get rights and content

Abstract

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.

Section snippets

Structure and function of cAMP-dependent protein kinase A

The binding of extracellular ligand to its G-protein-coupled receptor at the cell membrane activates adenylate cyclase, generating cAMP at discrete points along the membrane. cAMP binds the regulatory (R) subunits of protein kinase A (PKA), dissociating the holoenzyme and releasing free catalytic subunit (C-PKA). PKA-dependent phosphorylation of nuclear and cytoplasmic substrates controls multiple cell functions, including motility, metabolism, differentiation, synaptic transmission, ion

Intracellular targeting of protein kinase A

PKAII is concentrated in particulate membranes and cellular organelles through interaction with a family of distinct but functionally homologous A kinase anchor proteins (AKAPs) Rubin 1994, Edwards and Scott 2000, Dodge and Scott 2000. AKAPs immobilize PKAII isoforms at specific intracellular locations by binding RII subunits. Although the preferred ligand is RII/PKAII, several AKAPs also bind RI/PKAI (see below).

The first 30 amino acid residues of RII participate in AKAP binding, as shown by

Prototypic neuronal AKAP: AKAP150, AKAP79 and AKAP75

A single species of neuronal AKAP is expressed abundantly in the brain and highly conserved in mouse (150 kDa), bovine (75 kDa) and human (79 kDa) Bregman et al 1989, Hirsch et al 1992, Carr et al 1992, This prototypic neuronal AKAP targets PKAII to the postsynaptic cytoskeleton and perikarya of neurons. It is expressed in Purkinje cells and in olfactory bulb neurons, basal ganglia, cerebral cortex, and other forebrain regions. Most AKAP150 is concentrated in primary branches of dendrites in

Organelle-associated AKAPs

Over the last few years, new AKAP families have been identified in different species and tissues. A brief list of these AKAPs and the organelle to which they tether are shown in Table 2.

The role of AKAPs in development

AKAP-like proteins have been identified in lower organisms. In Caenorhabditis elegans, a single type of PKA mediates the activation and the flux of cAMP signaling (Lu et al., 1990). AKAP from C. elegans (AKAPCE) that binds RI-like (RCE) R-PKA with high affinity (apparent K_d 10 nM) has been cloned and characterized Angelo and Rubin 1998, Angelo and Rubin 2000. Although hydrophobic residues critical for the RII-binding affinity of mammalian AKAPs are conserved in AKAPCE, the R-CE-binding region

AKAPs bind other effectors

Although AKAPs were identified on the basis of their interaction with PKA, further structural and functional analysis indicated a more complex role for these proteins in signal transduction. As described above, AKAPs bind other signaling molecules, principally phosphatases and kinases. For example, AKAP79 binds PKC and calcineurin (CaN), both in vivo and in vitro Coghlan et al 1995, Klauck et al 1996. In cultured hippocampal neurons, PKAII co-localizes with PKC, CaN and AKAP in the dendritic

Dynamic assembly of the AKAP scaffold on membranes

Although the interaction between RII/PKAII and AKAP is known in some detail, the assembly of PKAII-AKAP-non-PKA effector complexes on membranes is still poorly understood. Since the phosphatases and kinases do not interact directly, AKAP functions as a nucleating core, concentrating diverse effector enzymes in close proximity to possible substrates. The stability of this multivalent scaffold is regulated by the activity of its components. For example, basic regions located at the NH₂ terminus

Local effects

The synthesis and accumulation of AKAP elicits a redistribution of endogenous PKA holoenzymes from a relatively large cytoplasmic volume to more restricted subcellular compartments. This anchors PKA in close proximity to target substrates. This optimizes local phosphorylation after PKA activation by agonist-induced elevations in intracellular cAMP. The released C subunits are recaptured locally by the R subunits after cAMP concentration decreases, thus restricting the effects of kinase

Acknowledgements

The work in the authors’ laboratories was supported by grants from “Associazione Italiana per la Ricerca sul Cancro (A.I.R.C.)“; Targeted Project Biotechnology CNR; and MURST (Italian Department of University and Research), the Lucille P. Markey Charitable Trust, POl CA23767-15 (NIH). We thank Dr C.S.Rubin for his support and helpful discussions. We acknowledge helpful comments and suggestions from Drs A Marx and L. Cardone. Special thanks are due to Mr F. D’Agnello for the artwork.