Functional aspects of calmodulin in protozoa

Abstract

Calmodulin is a ubiquitous endogenous calcium receptor in eukaryotic cells and is involved in many different physiological functions. The presence of calmodulins in protozoa has been reported for Tetrahymena and Paramecium. Calmodulin purified from T. pyriformis is composed of 147 amino acids and the amino-terminal is acetylated.Compared to bovine brain calmodulin, there are 11 substitutions and 1 deletion of amino acid residue(s). Besides such structural specificity, Tetrahymena calmodulin (T-CaM) is highly unique in that it has a potential ability to activate the pellicle-bound guanylate cyclase in a calcium-dependent manner. This stimulatory effect is also observed in Paramecium calmodulin (P-CaM). Bovine brain calmodulin represses the T-CaM-dependent activation of guanylate cyclase, indicating that bovine brain CaM exerts its inhibitory influence by interfering with the calmodulin-binding site of this enzyme. The 14S and 30S dyneins extracted from demembranated cilia with EDTA contai ATPase activities which are potentiated by addition of calmodulin. Taken together with its location in cilia, T-CaM appears to regulate the ciliary movement. Moreover, evidence has been provided that calmodulin may be implicated in the glycoprotein biosyn. The phosphorylated derivatives of the long chain polyisoprenoid alcohols (dolichol phosphates) are known to act as coenzymes, and the microsomal enzyme involving dolichol phosphorylation is calcium-requiring CTP-dependent and activated by calmodulin. A role for calmodulin in the control of cell growth is postulated by the concerted fluctuations of calmodulin contentand guanylate cyclase activity during the cell cycle in synchronized Tetrahymena cells. In addition, some other possible functions for calmodulin can be considered in protozoa.