Applications for mass spectrometry in the study of ion channel structure and function

Ion channels are intrinsic membrane proteins that form gated ion-permeable pores across biological membranes. Depending on the type, ion channels exhibit sensitivities to a diverse range of stimuli including changes in membrane potential, binding by diffusible ligands, changes in temperature and direct mechanical force. The purpose of these proteins is to facilitate the passive diffusion of ions down their respective electrochemical gradients into and out of the cell, and between intracellular compartments. In doing so, ion channels can affect transmembrane potentials and regulate the intracellular homeostasis of the important second messenger, Ca(2+). The ion channels of the plasma membrane are of particular clinical interest due to their regulation of cell excitability and cytosolic Ca(2+) levels, and the fact that they are most amenable to manipulation by exogenously applied drugs and toxins. A critical step in improving the pharmacopeia of chemicals available that influence the activity of ion channels is understanding how their three-dimensional structure imparts function. Here, progress has been slow relative to that for soluble protein structures in large part due to the limitations of applying conventional structure determination methods, such as X-ray crystallography, nuclear magnetic resonance imaging, and mass spectrometry, to membrane proteins. Although still an underutilized technique in the assessment of membrane protein structure, recent advances have pushed mass spectrometry to the fore as an important complementary approach to studying the structure and function of ion channels. In addition to revealing the subtle conformational changes in ion channel structure that accompany gating and permeation, mass spectrometry is already being used effectively for identifying tissue-specific posttranslational modifications and mRNA splice variants. Furthermore, the use of mass spectrometry for high-throughput proteomics analysis, which has proven so successful for soluble proteins, is already providing valuable insight into the functional interactions of ion channels within the context of the macromolecular-signaling complexes that they inhabit in vivo. In this chapter, the potential for mass spectrometry as a complementary approach to the study of ion channel structure and function will be reviewed with examples of its application.