Voltage-sensitive sodium and calcium channels play critical roles in the electrical excitability of nerve and muscle cells. These ion channels have been purified in functional form, their protein components have been characterized, and the role of their subunits in ion channel function has been studied. Each ion channel has a principal high molecular weight subunit which contains the main functional domains in association with additional smaller subunits whose functional roles are not yet established. Both channels are substrates for cAMP-dependent protein phosphorylation. The expression of the protein components of these channels is tissue-specific. 2. Analyses of the primary structure of the principal alpha subunit of the sodium channel have led to specific hypotheses for the structural basis of formation of a transmembrane ion channel and for voltage-dependent gating.
