Figure 5.
The S45/D mutant is only weakly activated by calmodulin. Membranes isolated from yeast expressing ACA2–1 (WT), ACA2–3 (S45/A), ACA2–4 (S45/D), or vector only were used to determine the initial rate of uptake (after first 40 s) as a function of calmodulin at a free [Ca2+] of 2.6 μM. The maximum initial activity of the wild-type enzyme was 5.8 nmol/min per mg. The activity of mutant enzymes was normalized to the same amount of wild-type enzyme. Average of three independent assays is shown.