Shining light on rhodopsin selectivity: How do proteins decide whether to transport H(+) or Cl(-)?

The versatile microbial rhodopsin family performs a variety of biological tasks using a highly conserved architecture, making it difficult to understand the mechanistic basis for different functions. Besaw et al. now report structures of a recently discovered cyanobacterial Cl(-)-pumping rhodopsin and its functionally divergent mutant that reveal how these transmembrane proteins create a gradient of activity with subtle changes. These insights are paralleled by a second recent report, which in combination answers long-standing questions about rhodopsin selectivity and will facilitate future engineering efforts.