Cell Cycle‐associated Changes of Guanylate Cyclase Activity in Synchronized Tetrahymena: a Possible Involvement of Calmodulin in its Regulation1

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Abstract
Guanylate cyclase activity decreased during the division phase of heat‐shock synchronized Tetrahymena pyriformis, strain GL. However, when Ca2+ was removed by EGTA to negate the effects of the Ca2+‐binding protein (calmodulin), which is required for the full activity of guanylate cyclase in this organism, no significant change in the enzymatic activity was observed throughout the cell cycle. On the other hand, the reduced guanylate cyclase activity at division phase was associated with a decreased level of calmodulin content. These results suggest that fluctuations in guanylate cyclase activity during the cell cycle would be dependent on the concentration of calmodulin.