SERCA, or sarco/endoplasmic reticulum Ca²⁺-ATPase, or SR Ca²⁺-ATPase, is a calcium ATPase-type P-ATPase, resides in the sarcoplasmic reticulum (SR) within muscle cells. It is a Ca²⁺ ATPase that transfers Ca²⁺ from the cytosol of the cell to the lumen of the SR at the expense of ATP hydrolysis during muscle relaxation. There are 3 major domains on the cytoplasmic face of SERCA: the phosphorylation and nucleotide-binding domains, which form the catalytic site, and the actuator domain, which is involved in the transmission of major conformational changes. It seems that, in addition to the calcium-transporting properties, SERCA1 generates heat in some adipocytes. SERCA is normally inhibited by the defective phosphorylation of protein phospholamban, with which it is closely associated. Another protein, calsequestrin, binds calcium within the SR and helps to reduce the concentration of free calcium within the SR, which assists SERCA so that it does not have to pump against such a high concentration gradient. The SR has a much higher concentration of Ca²⁺(10,000x) inside when compared to the intracellular Ca²⁺ concentration. The rate at which SERCA moves Ca²⁺ across the SR membrane can be controlled by phospholamban (PLB/PLN) under β-adrenergic stimulation. When PLB is associated with SERCA, the rate of Ca²⁺ movement is reduced, upon dissociation of PLB Ca²⁺ movement increases.