catalytic cycle (1:299). Finally, a narrow stalk region links the membranous region to the cytosolic head.
Electron micrographs of Ca2+-ATPase typically show an outline of a globule structure attached to the plasma bilayer by a narrow stalk. About two-thirds of the enzyme's mass is exposed on the cytoplasmic surface of the plasma bilayer; most of the remaining one-third is contained within the lipid phase (6:147-167). On freeze fracture the transmembrane helices show up as intramembranous particles. Researchers have suggested that "three extramembranous globular domains form a headpiece" which "lies on top of a stalk comprised of five helices which are, in turn, adjoined to five transmembrane helices (3:696-700)." These transmembrane helices then combine with five additional helices at the molecule's carboxy-terminal end; together, the ten helices form the ATPase's basepiece.
The SR Ca2+ pump differs from the plasma membrane Ca2+ pump by the fact that it does not have a calmodulin binding site. In the plasma membrane pump, this binding site is found near the enzyme's C terminus on the membrane's cytosolic side. In 1977, researchers demonstrated that calmodulin binding activates the plasma membrane ATPase (11:8070-8076). This generally results in an increased affinity for Ca2+, and an enhanced the maximal rate of Ca2+ pumping. In contrast, with the SR pump, regulation occurs through an associated SR membrane protein known as phospholamban (2:293-304).
In addition to their different regulatory mechanisms, plasma membrane and SR Ca2+-ATPases also have distinct functions. The plasma membrane pump transports calcium ions from the cytosol to the extracellular space; whereas, the SR enzyme transfers calcium ion into the SR lumen. Working together, the pumps provide cells with free cytosolic Ca2+ concentrations that are 1,000- to 10,000-fold less than those of the extracellular space (10:285-297).
Muscle con...