Mutational analyses identify a single amino acid critical for color tuning in proteorhodopsins

Microbial rhodopsins are light-activated proteins that contain seven transmembrane alpha-helices. Spectral tuning in microbial rhodopsins is a useful optogenetic tool. In this study, we report a new site that controls spectral tuning. In the proteorhodopsins ISR34 and ISR36, a single amino-acid substitution at Cys189 caused an absorption maximum shift of 44 nm, indicating spectral tuning at a specific site. Comparison of single amino acid substitutions was conducted using photochemical and photobiological approaches. The maximum absorption for red-shift was measured for mutations at positions 189 and 105 in ISR34, both residues being equally important. Structural changes resulting from amino acid substitutions are related to pK_a  values, pumping activity, and spectral tuning.

A Temporal Activity of CA1 Neurons Underlying Short-Term Memory for Social Recognition Altered in PTEN Mouse Models of Autism Spectrum Disorder

Memory-guided social recognition identifies someone from previous encounters or experiences, but the mechanisms of social memory remain unclear. Here, we find that a short-term memory from experiencing a stranger mouse lasting under 30 min interval is essential for subsequent social recognition in mice, but that interval prolonged to hours by replacing the stranger mouse with a familiar littermate. Optogenetic silencing of dorsal CA1 neuronal activity during trials or inter-trial intervals disrupted short-term memory-guided social recognition, without affecting the ability of being sociable or long-term memory-guided social recognition. Postnatal knockdown or knockout of autism spectrum disorder (ASD)-associated phosphatase and tensin homolog (PTEN) gene in dorsal hippocampal CA1 similarly impaired neuronal firing rate in vitro and altered firing pattern during social recognition. These PTEN mice showed deficits in social recognition with stranger mouse rather than littermate and exhibited impairment in T-maze spontaneous alternation task for testing short-term spatial memory. Thus, we suggest that a temporal activity of dorsal CA1 neurons may underlie formation of short-term memory to be critical for organizing subsequent social recognition but that is possibly disrupted in ASD.

Biliverdin Reductase-A Deficiency Brighten and Sensitize Biliverdin-binding Chromoproteins

Tissue absorbance, light scattering, and autofluorescence are significantly lower in the near-infrared (NIR) range than in the visible range. Because of these advantages, NIR fluorescent proteins (FPs) are in high demand for in vivo imaging. Nevertheless, application of NIR FPs such as iRFP is still limited due to their dimness in mammalian cells. In contrast to GFP and its variants, iRFP requires biliverdin (BV) as a chromophore. The dimness of iRFP is at least partly due to rapid reduction of BV by biliverdin reductase-A (BLVRA). Here, we established biliverdin reductase-a knockout (Blvra-/-) mice to increase the intracellular BV concentration and, thereby, to enhance iRFP fluorescence intensity. As anticipated, iRFP fluorescence intensity was significantly increased in all examined tissues of Blvra-/- mice. Similarly, the genetically encoded calcium indicator NIR-GECO1, which is engineered based on another NIR FP, mIFP, exhibited a marked increase in fluorescence intensity in mouse embryonic fibroblasts derived from Blvra-/- mice. We expanded this approach to an NIR light-sensing optogenetic tool, the BphP1-PpsR2 system, which also requires BV as a chromophore. Again, deletion of the Blvra gene markedly enhanced the light response in HeLa cells. These results indicate that the Blvra-/- mouse is a versatile tool for the in vivo application of NIR FPs and NIR light-sensing optogenetic tools.Key words: in vivo imaging, near-infrared fluorescent protein, biliverdin, biliverdin reductase, optogenetic tool.

Engineering an improved light-induced dimer (iLID) for controlling the localization and activity of signaling proteins

The discovery of light-inducible protein-protein interactions has allowed for the spatial and temporal control of a variety of biological processes. To be effective, a photodimerizer should have several characteristics: it should show a large change in binding affinity upon light stimulation, it should not cross-react with other molecules in the cell, and it should be easily used in a variety of organisms to recruit proteins of interest to each other. To create a switch that meets these criteria we have embedded the bacterial SsrA peptide in the C-terminal helix of a naturally occurring photoswitch, the light-oxygen-voltage 2 (LOV2) domain from Avena sativa. In the dark the SsrA peptide is sterically blocked from binding its natural binding partner, SspB. When activated with blue light, the C-terminal helix of the LOV2 domain undocks from the protein, allowing the SsrA peptide to bind SspB. Without optimization, the switch exhibited a twofold change in binding affinity for SspB with light stimulation. Here, we describe the use of computational protein design, phage display, and high-throughput binding assays to create an improved light inducible dimer (iLID) that changes its affinity for SspB by over 50-fold with light stimulation. A crystal structure of iLID shows a critical interaction between the surface of the LOV2 domain and a phenylalanine engineered to more tightly pin the SsrA peptide against the LOV2 domain in the dark. We demonstrate the functional utility of the switch through light-mediated subcellular localization in mammalian cell culture and reversible control of small GTPase signaling.