Calmodulin pump regulation

The binding of calcium ion to calmodulin, a major biochemical regulator of ion pumps and receptors, occurs on a time scale about a thousand times shorter than that observed for RNA conformational change. This Ca2+-calmodulin binding, which can be followed successfully by nuclear magnetic resonance (NMR), occurs in about ten milliseconds. 

Calcium effects. The biochemical effects of Ca "" in the cytoplasm are mediated by special Ca -binding proteins calcium sensors"). These include the annexins, calmodulin, and troponin C in muscle (see p. 334). Calmodulin is a relatively small protein (17 kDa) that occurs in all animal cells. Binding of four Ca "" ions (light blue) converts it into a regulatory element. Via a dramatic conformational change (cf 2a and 2b), Ca -calmodulin enters into interaction with other proteins and modulates their properties. Using this mechanism, Ca "" ions regulate the activity of enzymes, ion pumps, and components of the cytoskeleton. 

Activity is modulated by other proteins present in the membrane. These include a glycoprotein (MW 53 000) which stimulates ATPase activity 138 a 60 000 molecular weight protein, which is phosphorylated in a calmodulin-dependent fashion, affects accumulation of calcium 139 while the activity of the enzyme is affected by an endogenous kinase and phosphatase which phosphorylates and dephosphorylates the protein.140 Phospholamban is a proteolipid (MW 22 000) in cardiac SR which undergoes both cyclic AMP-dependent and calcium-calmodulin-dependent phosphorylation,141 but at different sites. All these proteins are probably involved in regulating the activity of the calcium pump. 

Schuh, K., Uldrijan, S., Telkamp, M., Rothlein, N., Neyses, L., 2001, The plasmamembrane calmodulin-dependent calcium pump a major regulator of nitric oxide synthase I. J Cell Biol 155, 201-205. 

Fig. 2. A schematic representation of some of the mechanisms by which Car fluxes across the plasma membrane are regulated. In the plasma membrane (the striped area) there are both influx (=>) and energy-dependent ( ) efflux pathways. Two mechanisms by which Ca2+ influx can be increased are via the actions of the intracellular messengers inositol 1,3,4,5-tetrakisphosphate, and cAMP generated via activation of specific classes of surface receptors (R, and R2) linked to specific N proteins which activate either phosphatidylinositol 4,5-bisphosphate (PIP,) hydrolysis or adenylate cyclase (AC). Additionally, influx can be increased either by a direct receptor-coupled event or by a membrane depolarization (not shown). A rise in the Ca2+ concentration in the domain just beneath the plasma membrane, [Ca2+Isin, can lead to an activation of the Ca2+ pump either via a direct calmodulin (CaM)-dependent mechanism, or indirectly via the activation of protein kinase C (CK). Additionally, in some cells, an increase in cGMP concentration also increases Ca2+ efflux (not shown), and in still others cAMP may stimulate Ca2 efflux.

Calcium ions are also transported into the cell by a pump, which is a Ca +-dependent ATPase. This pump is necessary because the calcium ion concentration is four orders of magnitude higher outside than inside living cells. Calmodulin regulates the level of calcium ions and hence the calcimn pump. When the calcium concentration decreases, calcium is dissociated from calmodulin and the calcium pump is inactivated. The structure of such a pump from the sarcoplasmic reticulum is reported at 8 A resolution. This pump couples ATP hydrolysis with cation transport. The protein contains 10 transmembrane helices. A distinct cavity was located that led to the putative calcium-binding site, suggesting a path for a calcium passage. 

Carafoli, E. (1984). Calmodulin-sensitive calcium-pumping ATPase of plasma membranes Isolation, reconstitution and regulation. Fed. Proc. 43, 3005-3010. 

The endoplasmic reticulum takes up Ca " " using the sarco(endo)plasmic reticulum ATPase (SERCA) pump (Figure 11.4). The SERCA pump of many types of muscle is regulated by a protein called phospholambin, which binds to SERCA both in its cytosolic and its transmembrane regions, maintaining the pump in an inactivated state when in its nonphosphorylated form, but detaches from the pump upon phosphorylation, presumably due to a conformational change. In this sense, the SERCA pump is like the PMCA pump, except that Ca -saturated calmodulin activates PMCA, whereas kinase-dependent phosphorylation is involved in SERCA activation (perhaps also dependent on calmodulin). 

B. Regulation of the SERCA Pump by Ca I Calmodulin-Dependent Protein Kinase... 

---