The mobility of calcium-trigger proteins and its function.

Trigger activity implies the transfer of the energy of a signal to some amplified (energy) response. Actions in cells, from calcium concentration changes to major protein reorganization are discussed here. The changes must be fast, so mobile polymers must be involved. The first step is the calcium on/off binding to its receptor, calmodulin, troponin C or a comparable protein. Calcium binding is to a loop, EF-hand, between helices. The structures and internal mobilities of these proteins are described using nuclear magnetic resonance and the temperature dependence of NMR shifts. It is suggested that these proteins illustrate a general working hypothesis that proteins made from interacting helices as opposed to beta-sheet proteins will have relatively easy internal main chain motions. Loops connecting the helices then provide particularly obvious read-out points, for example of the initial message of calcium binding. These and other regions of loose structure appear to be associated with highly charged sequences. The further transfer of the trigger message is to highly mobile sequences in troponin I, troponin T and tropomyosin.