Biophysics: Transporter in the spotlight

Abstract

Membrane transporter proteins switch between conformational states to move substrates across membranes. The transition between these states can now be studied using single-molecule experiments.

Figure 1. Conformation states of a membrane transporter

The LeuT protein adopts various conformational states as it transports the amino acid leucine across bacterial cell membranes. It consists of a pore containing two gates, the inner and outer gates. Four conformational states are shown here; the total number of states is unknown. a, The transport cycle begins with an outward-facing state, C_o, in which the outer gate is open but the inner gate is closed. b, The binding of a leucine molecule and a sodium ion at sites between the gates causes the outer gate to close, generating the occluded state (Occ). c, A second leucine molecule can then bind to another site just outside the outer gate. d, Next, LeuT opens its inner gate and loses all its substrates to form the inward-facing state, C_i. Closure of the inner and gate and opening of the outer gate returns LeuT to the C_o state. Zhao et al. have used a fluorescence-based, single-molecule technique to study the transition between C_i and C_o.