Mechanism for targeting the A-kinase anchoring protein AKAP18δ to the membrane

A-kinase anchoring proteins (AKAP) are a family of scaffolding proteins that target protein kinase A (PKA) and other signaling molecules to cellular compartments and thereby spatiotemporally define cellular signaling events. The AKAP18 family comprises AKAP18α, β, γ and δ. The δ isoform targets PKA and phosphodiesterase PDE4D to aquaporin-2 (AQP2)-bearing vesicles to orchestrate the acute regulation of body water balance. Therefore, AKAP18δ must adopt a membrane localization that seems at odds with (i) its lack of palmitoylation or myristoylation sites which tailor its isoforms AKAP18α and AKAP18β to membrane compartments, and (ii) the high sequence identity to the preferentially cytoplasmic AKAP18γ. Here we show that the electrostatic attraction of AKAP18δ′s positively charged amino acids to negatively charged lipids explains its membrane targeting. As revealed by fluorescence correlation spectroscopy, the binding constant of purified AKAP18δ fragments to large unilamellar vesicles (LUV′s) correlates (i) with the fraction of net negatively charged lipids in the bilayer and (ii) with the total amount of basic residues in the protein. Although distantly located on the sequence, these positively charged residues concentrate in the tertiary structure and form a clear binding surface. Thus, specific recruitment of the AKAP18δ-based signaling module to membranes such as those of AQP2-bearing vesicles must be achieved by additional mechanisms, most likely compartment-specific protein-protein interactions.