Loops and Gates

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Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 21
DOI: 10.1126/science.302.5642.21a

One of the critical concerns applicable to all membrane transport proteins is how to move the target molecule across the cell membrane without creating a nonspecific, potentially lethal, leak. The usual solution is to keep the transporter closed with a gate and to design a binding site to recognize the desired substrate; opening of the gate is then regulated, for example, by changing the transmembrane voltage, in the case of KvAP, or by binding of a proton, in the case of lacY.

Chen et al. have examined the mechanism for the maltose transporter, which consists of two ATP-binding subunits—the MalK dimer—and two integral membrane subunits—MalF and MalG. Crystal structures of the MalK dimer in open, semi-open, and closed conformations (which are governed by ATP hydrolysis) reveal a tweezers-like movement of the monomers. The reorientation of MalK2 with respect to the cytoplasmic surface of MalFG has the effect of exerting force at the point of contact, which is formed by the Q loops from MalK and the EAA loops from MalFG. Closing the cytoplasmic portions of MalFG would then lever open the external portions, enabling maltose to enter the transport cavity. Finally, hydrolysis of ATP would reverse the tweezers motion and allow maltose access to the cytoplasm. — GJC

Mol. Cell 12, 651 (2003).

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