Paramecium Calmodulin Mutants Defective in Ion Channel Regulation Can Bind Calcium and Undergo Calcium-Induced Conformational Switching

Abstract

Calmodulin (CaM) is an essential eukaryotic protein that binds calcium ions cooperatively at four EF-hand binding sites to regulate signal transduction pathways. Interactions between the apo domains of vertebrate CaM reduce the calcium affinities of sites I and II below their intrinsic values, allowing sequential opening of the two hydrophobic clefts in CaM. Viable domain-specific mutants of Parameciumcalmodulin (PCaM) differentially affect ion channels and provide a unique opportunity to dissect the roles of the two highly homologous half-molecule domains. Calcium binding induced an increase in the level of ordered secondary structure and a decrease in Stokes radius in these mutants; such changes were identical in direction to those of wild type CaM, but the magnitude depended on the mutation. Calcium titrations monitored by changes in the intrinsic fluorescence of Y138 in site IV showed that the affinities of sites III and IV of wild type PCaM were (i) higher than those of the same sites in rat CaM, (ii) equivalent to those of the same sites in PCaM mutants altered between sites I and II, and (iii) higher than those of PCaM mutants modified in sites III and IV. Thus, calcium saturation drove all mutants to undergo conformational switching in the same direction but not to the same extent as wild type PCaM. The disruption of the allosteric mechanism that is manifest as faulty channel regulation may be explained by altered properties of switching among the 14 possible partially saturated species of PCaM rather than by an inability to adopt two end-state conformations or target interactions similar to those of the wild type protein.