Figure 2. Cartoon representation to indicate how an ancillary subunit can alter binding site accessibility.

A, a channel with a binding site in the intracellular pore has to open before the drug can bind. Drug binding may be followed by a post-binding conformational change in the channel. B, if the channel shown in A has a subunit associated with it that can bind to the open pore, then, following pore opening, the subunit can block access of the drug to the binding site, thus altering binding site accessibility.