A designed ion channel
Chris Miller group (Brandeis University)
Chris Miller's group of Brandeis University studies
controlled movement of solutes across biological membranes, which is managed
by two types of integral membrane proteins - channels, and transporters.
Channels allow for passive diffusion down a thermodynamic gradient, whereas
transporters operate by a cycle of conformational changes, usually coupled to
energy sources to drive substrates thermodynamically uphill. Accordingly,
channel-mediated movement of substrates tends to be orders of magnitude
faster than that of transporters. The peculiar CLC family of proteins, which
allow the movement of Cl- across membranes in most organisms, has
members that are Cl- channels, and members that are
Cl-/H+ antiporters. Only the structure of the
antiporter has been previously determined. In this creative work, the Miller
group performed a kind of "mechanistic metamorphosis" to convert a
proton-coupled chloride "antiporter" protein into a chloride channel. They
wanted to see if some mutational surgery could "break" the antiporter, by
removing putative conformational "gates", and thus turn it into a
transmembrane continuous pathway selective for Cl ion. They were successful
in this endeavor, and using data obtained at GM/CA-CAT and at beamline X29 of
the NSLS, they were able to determine the structure of the mutant. The
artificially-designed channel is very narrow, and slow; the authors expect
natural CLC channels to have a significantly larger channel.
Figure: Images of the Cl-
conduits of the antiporter (left), and the doubly-ungated mutant (right),
visualized with the program HOLE.
Jayaram, H, Accardi, A, Wu, F, Williams, C, Miller, C. Ion permeation
through a Cl--selective channel designed from a CLC
Cl-/H+ exchanger, Proc. Natl. Acad. Sci. USA 105 (32),
11194-11199 (2008). DOI: 10.1073/pnas.0804503105.