Ky. Ling et al., NEW NOTE-LETHAL CALMODULIN MUTATIONS IN PARAMECIUM - A STRUCTURAL ANDFUNCTIONAL BIPARTITION HYPOTHESIS, European journal of biochemistry, 222(2), 1994, pp. 433-439
The mechanisms by which calmodulin coordinates its numerous molecular
targets in living cells remain largely unknown. To further understand
how this pivotal Ca2+-binding protein functions in vivo, we isolated a
nd studied nine new Paramecium behavioral mutants defective in calmodu
lin. Nucleotide sequences of mutant calmodulin genes indicated single
amino-acid substitutions in mutants cam(4)(E104K), cam(5)-1 (D95G), ca
m(6) (A102V), cam(7) (H135R), cam(14)-1 (G59S) and cam(15) (D50G). In
addition, we encountered a second occurrence of three identified subst
itutions; they are cam(1)-2 (S1O1F), cam(5)-2 (D95G) and cam(14)-2 (G5
9S). Most of these mutational changes occurred in sites that have been
highly conserved throughout evolution. Furthermore, most of these cha
nges were not among the amino acids known to interact with the basic a
mphiphilic peptides of calmodulin targets. Consistent with our previou
s finding [Kink, J. A., Maley, M. E., Preston R. R., Ling, K.-Y., Wall
en-Friedman, M. A., Saimi, Y. and Kung, C. (1990) Cell 62, 165-174], m
utants that under-reacted to certain stimuli (allele number above 10)
had substitutions in the N-tenninal lobe of calmodulin, and those that
over-reacted (below 10) had substitutions in the C-terrninal lobe. No
mutations were found in the central helix that connects the lobes. Th
us, through undirected in vivo mutation analyses of Paramecium, we dis
covered that each of the two lobes of calmodulin has a distinct role i
n regulating the function of a specific ion channel and eventually the
behavior of Paramecium. We, therefore, propose a hypothesis of functi
onal bipartition of calmodulin that reflects its structural bipartitio
n.