A-kinase anchoring proteins: protein kinase A and beyond

doi:10.1016/S0955-0674(99)00085-X

Current Opinion in Cell Biology

Volume 12, Issue 2, 1 April 2000, Pages 217-221

https://doi.org/10.1016/S0955-0674(99)00085-X Get rights and content

Abstract

Compartmentalization of kinases and phosphatases is a key determinant in the specificity of second messenger mediated signaling events. Localization of the cAMP-dependent protein kinase (PKA) and other signaling enzymes is mediated by interaction with A-kinase anchoring proteins (AKAPs). In the past year there have been many advances in our understanding of AKAPs, particularly in the field of the functional consequences of PKA anchoring.

Introduction

As the genome project approaches completion, it is becoming clear that there are more signal transduction proteins than was originally anticipated. A conservative estimate suggests that 20% of genes may encode transmembrane receptors, G protein subunits, signal-generating enzymes, protein kinases and protein phosphatases. In fact, Hunter’s original prediction [1] of ‘1001 kinases’ has been revised and estimates are now approaching 2000. With many kinase and phosphatase genes identified, the research community now is focusing on understanding how these enzymes orchestrate phosphorylation events inside cells. Much attention has been focused on the cAMP-dependent protein kinase (PKA), which becomes activated in response to cellular events that stimulate the synthesis of the second messenger, cAMP. Given that PKA is involved in numerous parallel signaling cascades, understanding the functional complexities of how the kinase is activated in the right place and at the right time inside cells is important. This specificity is achieved, in part, through the compartmentalization of PKA at different subcellular locations through interaction with A-kinase anchoring proteins (AKAPs).

AKAPs were first identified as proteins that co-purified with the PKA holoenzyme when isolated from tissues. Since then, a variety of gel overlay, interaction cloning, yeast two-hybrid and proteomic approaches have identified up to 25 unique AKAPs, some of which are members of complex gene families with numerous splice variants and isoforms (reviewed in [2]). There is no overall sequence similarity among different AKAPs; they represent a family of functionally-related molecules that are characterized by their interaction with type I or type II regulatory subunits (RI or RII) of the PKA holoenzyme. In addition to a defined R subunit binding site, AKAPs also possess unique targeting sequences that direct the PKA–AKAP complex to cellular compartments. Finally, some anchoring proteins have the ability to maintain signaling scaffolds by simultaneously associating with other kinases and phosphatases.

Section snippets

The PKA–AKAP interaction

Early work predicted that the R binding surface on AKAPs formed an amphipathic helix [3]. This has now been firmly established by Jennings and colleagues [4^•] in NMR studies which show that the hydrophobic side chains on one face of the helix are the principal binding determinants for the interaction with the R subunit dimer. Two recent studies have significantly advanced our understanding of the AKAP binding surface on PKA. Neutron and X-ray scattering experiments have defined the quaternary

Targeting

Specificity in PKA anchoring is achieved by specific targeting motifs that direct AKAPs through protein–protein interactions to structural elements, or through protein–lipid interactions to membranes. For example, the neuronal anchoring protein AKAP79 contains three non-contiguous basic regions that facilitate association with acidic phospholipids in the plasma membrane [14], AKAP15/18 is tethered to membranes via myristoylation and dual palmitoylation signals 15, 16 and mAKAP, an anchoring

Multivalent scaffolds

Perhaps the most interesting property of AKAPs is their simultaneous anchoring of other kinases and phosphatases (reviewed in [25]). The first example of such an AKAP that binds multiple enzymes was AKAP79, which interacts with PKA, PKC and the calcium/calmodulin-dependent phosphatase, PP2B [26]. Functional studies from a variety of laboratories have suggested that AKAP79 maintains these enzymes at sites close to the plasma membrane in order to coordinately regulate the phosphorylation state

Functional consequences of PKA anchoring

Although much is now understood about the molecular organization of AKAPs, the most important questions revolve around the in vivo function of each signaling complex. Heterologous expression of AKAP79, AKAP15/18 and AKAP-KL have implicated these anchoring proteins in modulation of ion channels through directing pools of kinases and phosphatases close to particular channel subunits (reviewed in [27]). To date, perhaps the most sophisticated mechanism of channel modulation is yotiao, an anchoring

Conclusions and perspectives

Although there is considerable information about the molecular interactions of AKAPs, several fundamental questions remain unanswered. For instance, it is clear that multiple AKAPs are targeted to the same subcellular compartment. This raises the question of whether there are redundancies in PKA anchoring or if individual anchoring proteins precisely direct their kinases and phosphatases to individual substrates. Curiously, both situations may exist. The physical association of the yotiao

Acknowledgements

We would like to thank members of the Scott lab, in particular Marcie Colledge, Iain Fraser, and Dario Diviani for critical reading of the manuscript and helpful discussions. AS Edwards was supported by National Institute of Health program project grant DK54441.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest
•• of outstanding interest

References (40)

T. Hunter
Protein kinases and phosphatases: the yin and yang of protein phosphorylation and signaling
Cell
(1995)
M. Colledge et al.
AKAPs: from structure to function
Trends Cell Biol
(1999)
D.W. Carr et al.
Interaction of the regulatory subunit (RII) of cAMP-dependent protein kinase with RII-anchoring proteins occurs through an amphipathic helix binding motif
J Biol Chem
(1991)
J. Zhao et al.
Quaternary structures of a catalytic subunit-regulatory subunit dimeric complex and the holoenzyme of the cAMP-dependent protein kinase by neutron contrast variation
J Biol Chem
(1998)
Y. Li et al.
Mutagenesis of the regulatory subunit (RIIβ) of cAMP-dependent protein kinase IIβ reveals hydrophobic amino acids that are essential for RIIβ dimerization and/or anchoring RIIβ to the cytoskeleton
J Biol Chem
(1995)
Z.E. Hausken et al.
Mutational analysis of the A-kinase anchoring protein (AKAP)-binding site on RII
J Biol Chem
(1996)
L.J. Huang et al.
Identification of a novel dual specificity protein kinase A anchoring protein, D-AKAP1
J Biol Chem
(1997)
K. Miki et al.
Identification of tethering domains for protein kinase A type I alpha regulatory subunits on sperm fibrous sheath protein FSC1
J Biol Chem
(1998)
R. Angelo et al.
Molecular characterization of an anchor protein (AKAP_CE) that binds the RI subunit (R_CE) of type I protein kinase A from Caenorhabditis elegans
J Biol Chem
(1998)
K. Miki et al.
Single amino acids determine specificity of binding of protein kinase A regulatory subunits by protein kinase A anchoring proteins
J Biol Chem
(1999)

Cited by (184)

Everything you ever wanted to know about PKA regulation and its involvement in mammalian sperm capacitation
2020, Molecular and Cellular Endocrinology
Citation Excerpt :
As previously mentioned, PKA has the potential to phosphorylate many putative substrates. To restrain the signaling cascade to a specific subcellular domain, PKA is tethered by its interaction with scaffolding proteins, called AKAPs (Table 2) (Edwards and Scott, 2000). Sequestering PKA to a specific subcellular environment not only ensures that the enzyme is in proximity to its targets, but also segregates this activity to prevent indiscriminate phosphorylation of other substrates.
The 3′, 5′-cyclic adenosine monophosphate (cAMP) dependent protein kinase (PKA) is a tetrameric holoenzyme comprising a set of two regulatory subunits (PKA-R) and two catalytic (PKA-C) subunits. The PKA-R subunits act as sensors of cAMP and allow PKA-C activity. One of the first signaling events observed during mammalian sperm capacitation is PKA activation. Thus, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. It is widely accepted that PKA specificity depends on several levels of regulation. Anchoring proteins play a pivotal role in achieving proper localization signaling, subcellular targeting and cAMP microdomains. These multi-factorial regulation steps are necessary for a precise spatio-temporal activation of PKA. Here we discuss recent understanding of regulatory mechanisms of PKA in mammalian sperm, such as post-translational modifications, in the context of its role as the master orchestrator of molecular events conducive to capacitation.
cAMP regulation of protein phosphatases PP1 and PP2A in brain
2019, Biochimica et Biophysica Acta - Molecular Cell Research
Normal functioning of the brain is dependent upon a complex web of communication between numerous cell types. Within neuronal networks, the faithful transmission of information between neurons relies on an equally complex organization of inter- and intra-cellular signaling systems that act to modulate protein activity. In particular, post-translational modifications (PTMs) are responsible for regulating protein activity in response to neurochemical signaling. The key second messenger, cyclic adenosine 3′,5′-monophosphate (cAMP), regulates one of the most ubiquitous and influential PTMs, phosphorylation. While cAMP is canonically viewed as regulating the addition of phosphate groups through its activation of cAMP-dependent protein kinases, it plays an equally critical role in regulating removal of phosphate through indirect control of protein phosphatase activity. This dichotomy of regulation by cAMP places it as one of the key regulators of protein activity in response to neuronal signal transduction throughout the brain. In this review we focus on the role of cAMP in regulation of the serine/threonine phosphatases protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) and the relevance of control of PP1 and PP2A to regulation of brain function and behavior.
Modeling and Analysis of Intracellular Signaling Pathways
2014, From Molecules to Networks: An Introduction to Cellular and Molecular Neuroscience: Third Edition
Sequences of biochemical reactions, termed signaling pathways, transmit synaptic or hormonal inputs to cytoplasmic or nuclear targets. Ca²⁺ influx, neurotransmitters, and growth factors rely on these pathways to drive long-term processes such as neuronal development or synaptic plasticity. We introduce the concepts and techniques necessary to analyze signaling pathways using concise mathematical language. Mathematical models are needed to represent and predict the response of pathways and neurons to stimulus patterns, because these systems are too complex for intuition alone. We focus on understanding qualitative models that illustrate how typical biochemical motifs can give rise to dynamics such as oscillations in reaction rates or switching between biochemical steady states. These models have shown how positive or negative feedback generates such dynamics. Positive feedback, and steady-state switching, may be essential to drive cellular differentiation and to maintain long-term memory. We also describe how to determine the sensitivity of model behavior to parameters, and to assess which parameters can be constrained by data, providing strong tests of a model. Characterization of coupled biochemical and genetic pathways, effectively using biochemical and gene expression data, requires collaboration between molecular biologists and modelers, to fit data into a mathematical framework and design experiments to test model predictions. Understanding these methods has become essential in order to predict and analyze responses of neurons and organisms to physiological and environmental stimuli, and to develop drugs that may treat disorders impairing neuronal biochemistry and cognitive function.
New insights into posttranslational modifications of proteins during bull sperm capacitation
2023, Cell Communication and Signaling
Plasma-Derived Exosome Proteins as Novel Diagnostic and Prognostic Biomarkers in Neuroblastoma Patients
2023, Cells
The Role of Cyclic Adenosine Monophosphate (cAMP) in Modulating Glucocorticoid Receptor Signaling and Its Implications on Glucocorticoid-Related Collagen Loss
2023, International Journal of Molecular Sciences

View all citing articles on Scopus

View full text

ReviewA-kinase anchoring proteins: protein kinase A and beyond

Abstract

Introduction

Section snippets

The PKA–AKAP interaction

Targeting

Multivalent scaffolds

Functional consequences of PKA anchoring

Conclusions and perspectives

Acknowledgements

References and recommended reading

Cell

Trends Cell Biol

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Neuron

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Neuron

Curr Biol

J Biol Chem

Mol Cell

J Biol Chem

Review
A-kinase anchoring proteins: protein kinase A and beyond