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 reversible 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 (USPs). 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.