Near infrared fluorescent biliproteins generated from bacteriophytochrome AphB of Nostoc sp. PCC 7120

Bacteriophytochromes - from informative model systems of phytochrome function to powerful tools in cell biology

Bacteriophytochromes are a subfamily of the diverse light responsive phytochrome photoreceptors. Considering their preferential interaction with biliverdin IXα as endogenous cofactor, they have recently been used for creating optogenetic tools and engineering fluorescent probes. Ideal absorption characteristics for the activation of bacteriophytochrome-based systems in the therapeutic near-infrared window as well the availability of biliverdin in mammalian tissues have resulted in tremendous progress in re-engineering bacteriophytochromes for diverse applications. At the same time, both the structural analysis and the functional characterization of diverse naturally occurring bacteriophytochrome systems have unraveled remarkable differences in signaling mechanisms and have so far only touched the surface of the evolutionary diversity within the family of bacteriophytochromes. This review highlights recent findings and future challenges.

Visible light biophotosensors using biliverdin from Antheraea yamamai

We report an endogenous photoelectric biomolecule and demonstrate that such a biomolecule can be used to detect visible light. We identify the green pigment abundantly present in natural silk cocoons of Antheraea yamamai (Japanese oak silkmoth) as biliverdin, using mass spectroscopy and optical spectroscopy. Biliverdin extracted from the green silk cocoons generates photocurrent upon light illumination with distinct colors. We further characterize the basic performance, responsiveness, and stability of the biliverdin-based biophotosensors at a photovoltaic device level using blue, green, orange, and red light illumination. Biliverdin could potentially serve as an optoelectric biomolecule toward the development of next-generation implantable photosensors and artificial photoreceptors.

Conversion of phycocyanobilin-binding GAF domain to biliverdin-binding domain

Cyanobacteriochromes (CBCRs) are biliprotein photoreceptors that only exist in cyanobacteria and have a broad spectral response range from ultra-violet to far-red. The red/green-type CBCRs can show red/green reversible photoconversion via a covalently bound phycocyanobilin (PCB). In recent years, several CBCRs binding with not only PCB but also biliverdin (BV) have been discovered, which raises the possibility of CBCRs being applied as optogenetic tools. Through molecular modification, we hope to engineer BV-binding CBCRs responsive to the near-infrared spectral region (650–900 nm), of which the red/green type of CBCRs are suitable resources for experimentation. Here, we use Slr1393g3 (the third GAF domain of a red/green photoswitching CBCR from Synechocystis sp. PCC 6803) as a template to perform such molecular evolution using both random mutagenesis and site-directed mutagenesis. After several rounds of random mutagenesis, we obtained several BV-binding variants of Slr1393g3. These BV-binding variants have a maximal absorbance at ̃690 nm and a fluorescence at ̃720 nm. Additionally, some of them have remarkable photochromicity between a far-red light-absorbing state and a red light-absorbing state. Based on the primary amino acid sequence and structural models, the Phe474 surrounding ring D of BV is thought as a crucial site for chromophore selectivity.

FRET in a Synthetic Flavin- and Bilin-binding Protein

The last decade has seen development and application of a large number of novel fluorescence-based techniques that have revolutionized fluorescence microscopy in life sciences. Preferred tags for such applications are genetically encoded fluorescent proteins (FP), mostly derivatives of the green fluorescent protein (GFP). Combinations of FPs with wavelength-separated absorption/fluorescence properties serve as excellent tools for molecular interaction studies, for example, protein-protein complexes or enzyme-substrate interactions, based on the FRET phenomenon (Förster resonance energy transfer). However, alternatives are requested for experimental conditions where FP proteins or FP couples are not or less efficiently applicable. We here report as a "proof of principle" a specially designed, non-naturally occurring protein (LG1) carrying a combination of a flavin-binding LOV- and a photochromic bilin-binding GAF domain and demonstrate a FRET process between both chromophores.

Advances in Imaging Techniques and Genetically Encoded Probes for Photoacoustic Imaging

Photoacoustic (PA) imaging is a rapidly emerging biomedical imaging modality that is capable of visualizing cellular and molecular functions with high detection sensitivity and spatial resolution in deep tissue. Great efforts and progress have been made on the development of various PA imaging technologies with improved resolution and sensitivity over the past two decades. Various PA probes with high contrast have also been extensively developed, with many important biomedical applications. In comparison with chemical dyes and nanoparticles, genetically encoded probes offer easier labeling of defined cells within tissues or proteins of interest within a cell, have higher stability in vivo, and eliminate the need for delivery of exogenous substances. Genetically encoded probes have thus attracted increasing attention from researchers in engineering and biomedicine. In this review, we aim to provide an overview of the existing PA imaging technologies and genetically encoded PA probes, and describe further improvements in PA imaging techniques and the near-infrared photochromic protein BphP1, the most sensitive genetically encoded probe thus far, as well as the potential biomedical applications of BphP1-based PA imaging in vivo.