A new mechanism of transport was proposed by Reyes, Ginter & Boudker in 2009, called the elevator mechanism of transport. Unlike the traditional alternating-access hypothesis, this mechanism has proposed a substrate binding site in a mobile transport domain. After substrate binding, the transport domain undergoes a vertical transition towards the cytoplasm with the bound substrate. This mechanism was first discovered in the archaeal glutamate transporter homologue, GltPh, and was recently proposed to exist in several other transporters from distinct families, including the SLC28 concentrative nucleoside transporter family. Double cysteine mutants of a human nucleoside transporter (hCNT3) were designed with the known inward-facing crystal structures and a repeat-swapped outward-facing model to capture the outward-facing state, which has not been solved. Two successfully cross-linked hCNT3 double cysteine mutants were generated, which provide evidence that the elevator-like movement of the transport domain is crucial for substrate transport in hCNT3. It is also promising that these double cysteine mutants can be crystallised in other conformations, providing a better understanding of how drugs targeting the nucleoside transporters are initially recruited, bind, and are transported into cells.
Reyes N, Ginter C, Boudker O. (2009). Transport mechanism of a bacterial homologue of glutamate transporters. Nature 462, 880-885.