In non-muscle cells, a small increase in the cytosolic concentration of Ca2 can have a dramatic effect on several physiological processes. The presence of a specific protein in the cytosol, however, activates a Ca2 ATPase that serves to return these elevated cytosolic Ca2 levels back to basal levels, thereby maintaining the cell's normal physiology. Which protein is directly involved in activating this Ca2 ATPase

calmodulin

Explanation:

The calcium-modulated protein (calmodulin) is a calcium-binding receptor protein that modulates contractile proteins (i.e., actin and myosin proteins) of the skeletal muscle and non-muscle cells (e.g., platelets). Calmodulin binds to calcium ions (Ca2+) and subsequently activates a number of Ca2+ dependent enzymes (e.g., kinases or phosphatases), which finally activate/deactivate proteins in the calcium signal transduction pathway. Ca2+-ATPase pumps in the membranes of eukaryotic cells release Ca2+ from the cytoplasm and they are autoinhibited by low Ca2+ levels, while calmodulin-binding releases this autoinhibition and thus activates the pumps. In non-muscle cells such as platelets, calmodulin also mediates Ca2+ control of actin-myosin interaction by phosphorylation of the myosin light chain (MLC).

d.

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