Working against the current to transport material through cell membranes

Specific membrane proteins, called transporters, facilitate the movement of ions through cell walls. Ions flow from high to low concentrations spontaneously, but moving material in the opposite direction requires a source of free energy.

Primary active transporters can use the free energy from chemical reactions or light to move molecules against the natural flow. Secondary active transporters use the ionic gradient established by their primary companions to grab and drag along another material that would not move on its own.

Beckstein and Naughton described a big picture view of secondary active transporters, which provides a quantitative model of the whole transport cycle. They identified the broad principles observed in most of the proteins—although every rule in biology has its exception.

Secondary active transporters change between two conformations, one open to the outside and one that faces inward. The proteins undergo cyclic changes between these conformations to move materials across the cell membrane. For many transporters, the two conformations originate in an internal symmetry that is not immediately obvious.

“In the end, we put all of this together as one model where we see how the transporter moves between these different states,” said author Oliver Beckstein.

In certain scenarios, transporters should not undergo conformation changes because it would create a short circuit. Somehow, the proteins know when it is safe to shift and when it is not.

“We want to look at transporters individually to see what is stopping short circuits,” said author Fiona Naughton. “That’s something that is kind of easier to do on a case-by-case basis, as each one is slightly different.”

Source: “General principles of secondary active transporter function,” by Oliver Beckstein and Fiona Naughton, Biophysics Reviews (2022). The article can be accessed at https://doi.org/10.1063/5.0047967 .