Optobiology: optical control of biological processes via protein engineering

Abstract

Enabling optical control over biological processes is a defining goal of the new field of optogenetics. Control of membrane voltage by natural rhodopsin family ion channels has found widespread acceptance in neuroscience, due to the fact that these natural proteins control membrane voltage without further engineering. In contrast, optical control of intracellular biological processes has been a fragmented effort, with various laboratories engineering light-responsive properties into proteins in different manners. In the present article, we review the various systems that have been developed for controlling protein functions with light based on vertebrate rhodopsins, plant photoregulatory proteins and, most recently, the photoswitchable fluorescent protein Dronpa. By allowing biology to be controlled with spatiotemporal specificity and tunable dynamics, light-controllable proteins will find applications in the understanding of cellular and organismal biology and in synthetic biology.

Figure 1

Optical control of G protein signaling by engineered vertebrate rhodopsins. Opto-α₁-AR (chimera of green-absorbing rhodopsin and α₁ adrenergic receptor) activates adenylate cyclase via G_q, while Opto-β₂-AR (chimera of green-absorbing rhodopsin and β₂ adrenergic receptor) activates phospholipase C via G_s. The rhodopsin chromophore is the cofactor retinaldehyde. AC, adenylate cyclase; PLC, phospholipase C.

Figure 2

Systems for optical control of proteins based on plant photosensory proteins using chemical cofactors. (a) Light-induced heterodimerization by the LOV domain of FKF1 and light-induced conformational changes in the LOV domain of phototropin has been used to control protein localization, transcription, and the activity of a fused protein domain. The chromophore of LOV domains is FMN. (b) Light-induced heterodimerization or aggregation by cryptochromes has been used to control protein localization, transcription, protein fragment complementation, and signaling protein activity. The chromophore of cryptochromes is FAD. (c) Light-induced heterodimerization by phytochromes has been used to control protein localization and transcription. The chromophore of phytochromes is phytochromobilin. POI, protein of interest; DNABD, DNA-binding domain; TAD, transcriptional activation domain; NT frag, N-terminal protein fragment; CT frag, C-terminal protein fragment. The right-angle blue arrow indicates transcription, while the diagonal blue arrow indicates downstream protein activities.

Figure 3

Systems for optical control of proteins that do not require chemical cofactors. (a) Light-induced heterodimerization by UVR8 has been used to control transcription and protein localization. (b) Bidirectional light-controlled switching between tetrameric and monomeric states in Dronpa mutants, or between dimeric and monomeric states, has been used to control protein localization and the activity of single-chain fusion proteins. NLS, nuclear localization sequence.