A GFP-Based Sensor to Detect Transiently Expressed Proteins

Abstract

Green fluorescent protein (GFP) fusion tags are commonly used to study protein expression and cellular localization in vivo. But, GFP must undergo an autogenic post-translational modification, known as chromophore maturation, to become fluorescent, a process that can have a half-time longer than 30 minutes inside research model organisms. The timescale of chromophore maturation in GFP is thus slower than many key biological processes, limiting its usefulness in measuring those processes. In this thesis, we discuss the creation and engineering of a sensor for transiently expressed proteins (STEP) based on a fully matured but dim GFP. Upon specific binding of STEPtag, a small (15.5 kDa) protein to the sensor, full fluorescence is restored. Thus, by genetically fusing STEPtag to a protein of interest, it can be detected as soon as folding is complete, without any maturation delay. Through a combination of rational design and targeted directed evolution, we describe the improvement of the original sensor, gSTEP0, into an optimized version, gSTEP1. The sensor has been validated in vitro and in E. coli cells, and we have found that for gSTEP1, the fluorescence signal increases more than three-fold upon binding, with a Kd of 120 ± 30 nM and a kon of 1.7 x 105 M-1s-1, allowing detection of the protein of interest on the second timescale. We have also created a yellow version of the biosensor, and provide preliminary attempts at developing orthogonal binding pairs, as well as red- and cyan-coloured STEPs, which could eventually be used in multiplex experiments. Our biosensor opens the door to the study of short-timescale processes in research model organisms, such as Drosophila and zebrafish embryogenesis, as well as in host-pathogen interactions, which we are currently investigating.