1.A.50 The Phospholamban (Ca²⁺-channel and Ca²⁺-ATPase Regulator) (PLB) Family

Phospholamban (PLB) is the major phosphorylatable transmembrane protein of cardiac sarcoplasmic reticulum. It is 52 amino acyl residues long, and has been sequenced and characterized from mammals, the puffer fish, Tetraodon nigroviridis, and the chicken, Gallus gallus. Residues 1-31 (domains 1A (1-20) and 1B (21-31) are localized to the cytoplasm, while residues 32-52 (domain II) are predicted to span the membrane. It can be phosphorylated by protein kinases on residues 16 and 17. It assembles into a homopentameric complex in the native cardiac sarcoplasmic reticulum (SR) where it inhibits the activity of the P-type Ca²⁺ ATPase (TC #3.A.3) found in these membranes by decreasing its energetic efficiency. The pentameric (not the monomeric) PLB is necessary for the regulation of the Ca²⁺ ATPase and for myocardial contractility in vivo. PLB domain IA is the phosphorylation domain, and PLB domain IB interacts with the loop between TMSs 6 and 7 in the SR Ca²⁺-ATPase (Asahi et al., 2001). Binding to the ATPase causes structural changes in PLB (Hughes and Middleton, 2003).

Phospholamban has been shown to form cation-selective channels in lipid bilayers, with Ca²⁺ being transported in preference to K⁺ (Kovacs et al., 1988). It spontaneously opens and closes, and the transmembrane region, residues 26-52, is sufficient for channel activity. The putative regulatory portion of the protein, residues 2-25, did not form a channel. Possibly phospholamban regulates sarcoplasmic reticular Ca²⁺ flux by acting as a Ca²⁺ channel. However, channel activity is controversial (Becucci et al., 2009). Heparin-derived oligosaccharides (HDOs) interact with the cytoplasmic domain of PLB and consequently stimulate SERCA activity (Hughes et al., 2010).

Phosphorylation of PLB abolishes its inhibitory effect and therefore promotes Ca²⁺ transport into the SR lumen, enhancing cardiac relaxation. Phosphorylation occurs in response to β-adrenergic agonists. Pentamerization is believed to be mediated via the transmembrane domain of PLB, and phosphorylation may control the monomer-pentamer transition. Thus, PLB is a major regulator of the SR Ca²⁺ ATPase and of cardiac contractility, and phosphorylation may provide the primary mechanism for the control of these biochemical and physiological activities. Evidence suggests that one face of the PLB transmembrane helix interacts with helix M6 to cause inhibition.

In lipid bilayers, PLN adopts a pinwheel topology with a narrow hydrophobic pore, which excludes ion transport. In the T state, the cytoplasmic amphipathic helices (domains Ia) are absorbed into the lipid bilayer with the transmembrane domains arranged in a left-handed coiled-coil configuration, crossing the bilayer with a tilt angle of approximately 11° with respect to the membrane normal (Verardi et al., 2011). The tilt angle difference between the monomer and pentamer is approximately 13°. Thus, both topology and function of PLN are shaped by the interactions with lipids.

Sarcolipin is a 31 aa protein expressed in cardiac and skeletal muscle. It has hydrophilic N- and C-termini flanking a hydrophobic putative TMS. It negatively regulates the sarcoplasmic reticulum (SR) Ca²⁺ ATPase (SERCA) which transports Ca²⁺ into the SR, the contraction-relaxation cycle of the heart (Babu et al., 2006). The rate of and amount of Ca²⁺ transported into the SR determines both the rate of muscle relaxation and the Ca²⁺ load available for the next cycle of contraction. Sarcolipin inhibits SERCA as does phospholamban (TC #1.A.50) which also functions as a Ca²⁺ channel (Babu et al., 2006). Sarcolipin reduces Ca²⁺ transport by the skeletal muscle sarcoplasmic reticulum Ca²⁺-ATPase and results in heat generation (Mall et al., 2006). Possibly the interaction of sarcolipin with the Ca²⁺-ATPase is important for thermogenesis. Conserved tyrosyl residues in sarcolipin are directly involved in the inhibition of SERCA (Hughes et al., 2007).

Sarcolipin is 73% identical, 86% similar to the C-terminus of a protein from Danio revio of 1066 aas termed protocadherin-1-like protein (XM_690233). This protein is 63% identical and 75% similar to human protocadherin-1 (Q08174; 1026 aas), but not in the C-terminal region where the former protein is similar to sarcolipin. Structural similarities between sarcolipin and phospholamban suggest that they are homologous. In fact, the transmembrane regions of these two proteins exhibit 40% identity and 95% similarity.

Sarcolipin:
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Phospholamban:

Sarcolipin (SLN) forms channels selective toward chloride and phosphate ions when incorporated in a bilayer lipid membrane. ATP increases conductivity, and the dependence of the conductivity on the ATP concentration satisfies the Michaelis-Menten equation, with an association constant of 0.1 μM. Phenylphosphonium ion and adenosine monophosphate exert inhibitory effects on membrane permeabilization to phosphate by ATP if they are added before ATP, but not if they are added after it (Becucci et al., 2009). Thus, SLN acts as an ATP-induced phosphate carrier.