Cell biology. Going global on ubiquitin

Long AKAP18 isoforms anchor ubiquitin specific proteinases and coordinate calcium reuptake at the sarcoplasmic reticulum

Subcellular targeting of signaling enzymes influences where and when various modes of intracellular communication operate. Macromolecular complexes of signal transduction and signal termination elements favor reversable control of repetitive processes. This includes adrenergic stimulation of excitation-contraction coupling in the heart. Long isoforms of A-kinase anchoring protein 18 (AKAP18γ and δ) modulate this process via regulation of calcium uptake into the sarcoplasmic reticulum through the Ca2+ATPase 2a (SERCA2a). AKAP18 proximity-proteomic screening in cardiomyocytes identifies networks for protein kinase A (PKA) and ubiquitin-specific proteinases (USP's). A 2'phosphoesterase domain on AKAP18 interfaces with the USP4 isoform at the Z bands of sarcomeres. PKA stimulates USP4 activity in the presence of the anchoring protein. AKAP18 anchored PKA phosphorylates serine 829 on USP4, a conserved residue near the active site of this deubiquitinase. Antibodies against the pSer829 motif show that adrenergic stimulation enhances phosphorylation of USP4 in mouse adult cardiomyocytes. In related studies, elevated USP4 phosphorylation at Ser829 is detected in human post-myocardial infraction tissue as compared to healthy tissue. Thus, phosphorylation of sarcoplasmic USP4 may be a cardioprotective response. Pharmacological inhibition of PKA or deletion of the AKAP7/18 gene in mice decreases calcium flux through the exchanger. This suggests that loss of the anchoring protein impacts SERCA2 action. Thus, AKAP18/PKA/USP4 complexes are well positioned to influence the rate and magnitude of calcium reuptake during the cardiac cycle.

Single nucleotide polymorphisms alter kinase anchoring and the subcellular targeting of A-kinase anchoring proteins

A-kinase anchoring proteins (AKAPs) shape second-messenger signaling responses by constraining protein kinase A (PKA) at precise intracellular locations. A defining feature of AKAPs is a helical region that binds to regulatory subunits (RII) of PKA. Mining patient-derived databases has identified 42 nonsynonymous SNPs in the PKA-anchoring helices of five AKAPs. Solid-phase RII binding assays confirmed that 21 of these amino acid substitutions disrupt PKA anchoring. The most deleterious side-chain modifications are situated toward C-termini of AKAP helices. More extensive analysis was conducted on a valine-to-methionine variant in the PKA-anchoring helix of AKAP18. Molecular modeling indicates that additional density provided by methionine at position 282 in the AKAP18γ isoform deflects the pitch of the helical anchoring surface outward by 6.6°. Fluorescence polarization measurements show that this subtle topological change reduces RII-binding affinity 8.8-fold and impairs cAMP responsive potentiation of L-type Ca²⁺ currents in situ. Live-cell imaging of AKAP18γ V282M-GFP adducts led to the unexpected discovery that loss of PKA anchoring promotes nuclear accumulation of this polymorphic variant. Targeting proceeds via a mechanism whereby association with the PKA holoenzyme masks a polybasic nuclear localization signal on the anchoring protein. This led to the discovery of AKAP18ε: an exclusively nuclear isoform that lacks a PKA-anchoring helix. Enzyme-mediated proximity-proteomics reveal that compartment-selective variants of AKAP18 associate with distinct binding partners. Thus, naturally occurring PKA-anchoring-defective AKAP variants not only perturb dissemination of local second-messenger responses, but also may influence the intracellular distribution of certain AKAP18 isoforms.

Signalling scaffolds and local organization of cellular behaviour

Cellular responses to environmental cues involve the mobilization of GTPases, protein kinases and phosphoprotein phosphatases. The spatial organization of these signalling enzymes by scaffold proteins helps to guide the flow of molecular information. Allosteric modulation of scaffolded enzymes can alter their catalytic activity or sensitivity to second messengers in a manner that augments, insulates or terminates local cellular events. This Review examines the features of scaffold proteins and highlights examples of locally organized groups of signalling enzymes that drive essential physiological processes, including hormone action, heart rate, cell division, organelle movement and synaptic transmission.

Cell signaling in space and time: where proteins come together and when they're apart

Signal transduction can be defined as the coordinated relay of messages derived from extracellular cues to intracellular effectors. More simply put, information received on the cell surface is processed across the plasma membrane and transmitted to intracellular targets. This requires that the activators, effectors, enzymes, and substrates that respond to cellular signals come together when they need to.