DEVELOPMENT OF SINGLE-DOMAIN NEAR-INFRARED FLUORESCENT PROTEIN GAF-FP BASED ON BACTERIAL PHYTOCHROME

Fluorescent proteins (FPs) are widely used as genetically encoded markers for noninvasive and quantitative study of biological processes. Development of biomarkers that fluoresce in the near-infrared spectral range allows the study of animals at a deeper level due to high permeability of tissues to light in this wavelength range, compared to the visible light. For widespread use of FPs, such properties as low molecular weight and the monomer become important. In this paper, we developed a FP called the GAF-FP and based on the chromophore- binding domain of bacterial phytochrome from Rhodopseudomonas palustris (RpBphP1). GAF-FP has a molecular mass of ~ 19 kDa, 2 times lower than that of other FP based on BphPs and 1.4 times less than the commonly used GFP-like proteins. Unlike most other near-infrared FP, GAF-FP is a monomer, has high photostability and its structure can withstand the introduction of small peptide inserts. Moreover, GAF-FP can covalently bind two different tetrapyrrole chromophores: phycocyanobilin (PCB) and biliverdin (BV), which is found in mammalian tissues. GAF-FP with BV as a chromophore (GAF-FP—BV) has a main absorption band with a maximum at 635 nm and fluorescence maximum at 670 nm, whereby GAF-FP has a high signal to background ratio even if localized at a depth of several mm below the tissue surface. Apart from the near-infrared absorption band, GAF-FP—BV also has also an absorption band in the violet spectral range with a maximum at 378 nm. This property has been used by us to create a chimeric protein consisting of a modified luciferase from Renilla reniformis (RLuc8) and GAF-FP. We have shown that the chimeric protein is capable of resonance energy transfer from the substrate, which is oxidized by luciferase, to chromophore of GAF-FP—BV. In the absence of energy acceptor, RLuc8 catalyzes the cleavage of the substrate with light radiation having a peak of 400 nm. At the same time, as a part of GAF-FP—RLuc8 chimeric protein, the energy from the substrate is transferred to the chromophore of FP and then emitted in the near-infrared spectral range corresponding to GAF-FP fluorescence. These results open the way for the creation of new small near-infrared FPs based on various natural BphPs with a prospect of their wider use in cell and molecular biology.