Ratiometric fluorescence-imaging assays of plant membrane traffic using polyproteins

Fluorescent protein markers are widely used to report plant membrane traffic; however, effective protocols to quantify fluorescence or marker expression are lacking. Here the 20 residue self-cleaving 2A peptide from Foot and Mouth Disease Virus was used to construct polyproteins that expressed a trafficked marker in fixed stoichiometry with a reference protein in a different cellular compartment. Various pairs of compartments were simultaneously targeted. Together with a bespoke image analysis tool, these constructs allowed biosynthetic membrane traffic to be assayed with markedly improved sensitivity, dynamic range and statistical significance using protocols compatible with the common plant transfection and transgenic systems. As marker and effector expression could be monitored in populations or individual cells, saturation phenomena could be avoided and stochastic or epigenetic influences could be controlled. Surprisingly, mutational analysis of the ratiometric assay constructs revealed that the 2A peptide was dispensable for efficient cleavage of polyproteins carrying a single internal signal peptide, whereas the signal peptide was essential. In contrast, a construct bearing two signal peptide/anchors required 2A for efficient separation and stability, but 2A caused the amino-terminal moiety of such fusions to be mis-sorted to the vacuole. A model to account for the behaviour of 2A in these and other studies in plants is proposed.