Figure 4.

Allostery of photoreceptors. Allosteric signal transduction mechanisms realized in natural and engineered photoreceptors are grouped into non-associating and associating categories. Within the category of non-associating photoreceptors (Section Non-associating Photoreceptors and Optogenetic Applications), a disparate subclass of receptors relies on light-induced transitions of tertiary and quaternary (other than association) structure to regulate biological activity (Section Light-regulated Tertiary and Quaternary Structural Transitions). A second subclass capitalizes on light-induced order-disorder transitions, epitomized by Jα helix unfolding in AsLOV2 (Harper et al., 2003), most often to restrict access to either an effector active site or to a peptide epitope in light-regulated manner (Section Light-regulated Order-disorder Transitions). Within the category of associating photoreceptors (Section Associating Photoreceptors and Optogenetic Applications), light-induced signal transduction goes along with a change in oligomeric state. Association can be either light-promoted or light-repressed; can be between alike (homo) or different (hetero) partners; and can be of dimeric or higher stoichiometry. Note that other categorization schemes are conceivable as well; in particular, several photoreceptors based on light-induced Jα unfolding subsequently undergo heteroassociation (Lungu et al., 2012; Strickland et al., 2012; Guntas et al., 2015) (indicated by dashed arrow). Orange and yellow symbols denote T and R states, respectively, of photosensor modules; red symbols denote effector modules; and blue stars indicate effectors in their high-activity state.