Figure 4.

Proposed mechanisms of CFTR channel opening and closing. (a) Cartoon functional model of the pore in the channel closed state. The pore is wide open to the cytoplasm, but closed close to its extracellular end. (b) This cartoon model is somewhat reminiscent of the inward-facing structure of inactive CFTR (putative pore-lining amino acid side chains shown in red). (c) In the near-open structure, cytoplasmic access to the pore is via a lateral portal. (d) Cartoon functional model of the open channel pore incorporating a single lateral portal on the cytoplasmic side, a relatively wide inner vestibule, and narrow selectivity filter region close to the extracellular end of the pore. See [23] for more details concerning functional models of the open channel pore.