PLB resides in the SR membrane and is composed of just 52 amino acids. Its interaction with SERCA2a inhibits the pump’s ability to transport Ca2+ into the SR. However, this inhibitory action is repressed when PLB is phosphorylated at serine 16 by protein kinase A (PKA) during β-adrenergic signalling. In such instances, the rate at which SERCA2a transports Ca2+ across the SR membrane is greatly enhanced, thus making PLB a key regulator of cardiac function.
PLB is palmitoylated at Cys36 by zDHHC16. PLB palmitoylation promotes its phosphorylation by PKA and consequently increases cardiac output. Knocking out zDHHC16 in mice results in a diminished cardiac output as a consequence of bradycardia and a reduced stroke volume. zDHHC16 knockout (KO) mice also exhibit cardiomyopathy , indicating that palmitoylation of PLB and possibly other cardiac proteins by zDHHC16 plays a significant role in maintaining cardiac structure and function.
Ryanodine receptor (RyR2)
Ryanodine receptors are enormous tetrameric proteins composed of approximately 20¦000 amino acids that reside within the membrane of the SR. RyR2, the cardiac isoform, releases Ca2+ from the SR store in response to Ca2+ influx, usually via the opening of adjacent LTCCs. Mutations in the gene encoding the RyR2 receptor result in conditions such as familial polymorphic ventricular tachycardia , which can result in sudden cardiac death.
RyR2 is palmitoylated in the heart , but the functional consequences of this are not yet known. Interestingly RyR1, the isoform present in skeletal muscle, is palmitoylated at 18 cysteine residues, which reduces its activity and reduces stimulus-coupled Ca2+ release . Based on this, palmitoylation of the cardiac isoform is likely to play a key regulatory role in RyR2 receptormediated Ca2+ release from the SR. If the functional impact of RyR2 palmitoylation is anything like that of RyR1, mutations at or near the palmitoylation sites or mis-regulation of the enzymes responsible for controlling RyR2 palmitoylation would contribute to cardiac arrhythmias and therefore represent a novel therapeutic target in cardiac arrhythmogenesis.
Na+/Ca2+ Exchanger (NCX1)
NCX1 is a sarcolemmal ion transporter which mediates the bidirectional movement of calcium ions across the cell membrane. In forward mode, NCX1 extrudes one Na+ ion from the cell in exchange for three Ca2+ ions, and therefore controls cardiac relaxation. Because NCX1 is a passive exchanger, the mode in which it operates (forward = Ca2+ extrusion, reverse = Ca2+ influx) is dependent on multiple variables: the sodium gradient, the membrane potential and the subsarcolemmal free calcium concentration.
Several in vivo studies have advanced the understanding of NCX1’s contribution to cardiac function. Global KO of NCX1 in mice is embryonically lethal , but cardiac-specific NCX1 re-expression is insufficient for embryonic survival , therefore indicating that NCX1 expression out-with the embryonic heart is critical to development. Remarkably, a cardiac-specific NCX1 KO model demonstrated a very modest 20–30% reduction in cardiac contractility due to adaptive responses elsewhere in the myocardium . Increased NCX1 activity is implicated in numerous pathological conditions including heart failure , myocardial ischemia/reperfusion injury  and arrhythmogenesis . It may therefore be possible to target NCX1 for therapeutic benefit in cardiac disease.
NCX1.1 (the predominant splice variant in cardiac muscle) is palmitoylated at Cys739 within the intracellular loop. NCX1.1 activity is controlled by the availability of phosphatidylinositol 4,5-bisphosphate (PIP2); in the absence of PIP2, NCX1 inactivates. Palmitoylation of NCX1 is not only required for its inactivation but hastens the speed at which the exchanger inactivates . Approximately 60% of NCX1 is palmitoylated in ventricular muscle , suggesting that palmitoylation creates functionally different subpopulations of NCX1 in the heart. Palmitoylation does not control NCX1 trafficking to the cell membrane, nor does it have a profound effect on either the forward and reverse modes . However, by changing the sensitivity of NCX1 to inactivation, palmitoylation does ultimately change NCX1-mediated calcium fluxes in the cell . The role of NCX1 palmitoylation in cardiac function remains unreported; however, our research group is currently investigating this role by conducting a range of biochemical and physiological studies using a novel transgenic mouse model expressing a form of NCX1 that cannot be palmitoylated in the heart.