Photoreceptors Take Charge: Emerging Principles for Light Sensing

TLDR

Biological signaling begins with ligand‑induced changes in receptor protein states, and photoreceptors enable detailed mechanistic studies of these changes with high temporal resolution. The study critically evaluates proton and electron transfer mechanisms in photosensory proteins and identifies shared light‑sensing principles across six photoreceptor families. The authors review proton and electron transfer pathways and compare signaling mechanisms across rhodopsins, phytochromes, photoactive yellow proteins, LOV proteins, flavin‑based blue‑light sensors, and cryptochromes. Ultrafast primary photochemistry is followed by slower proton transfer steps that trigger large protein conformational changes during signaling state formation.

Abstract

The first stage in biological signaling is based on changes in the functional state of a receptor protein triggered by interaction of the receptor with its ligand(s). The light-triggered nature of photoreceptors allows studies on the mechanism of such changes in receptor proteins using a wide range of biophysical methods and with superb time resolution. Here, we critically evaluate current understanding of proton and electron transfer in photosensory proteins and their involvement both in primary photochemistry and subsequent processes that lead to the formation of the signaling state. An insight emerging from multiple families of photoreceptors is that ultrafast primary photochemistry is followed by slower proton transfer steps that contribute to triggering large protein conformational changes during signaling state formation. We discuss themes and principles for light sensing shared by the six photoreceptor families: rhodopsins, phytochromes, photoactive yellow proteins, light-oxygen-voltage proteins, blue-light sensors using flavin, and cryptochromes.