Hierarchized phosphotarget binding by the seven human 14-3-3 isoforms

Abstract

The seven human 14-3-3 isoforms, highly similar yet encoded by distinct genes, are among the top 1% highest-expressed human proteins. 14-3-3 proteins recognize phosphorylated motifs within numerous human or viral proteins. We analyzed by crystallography, fluorescence polarization, mutagenesis and fusicoccin-mediated modulation the structural basis and druggability of 14-3-3 binding to four E6 oncoproteins of tumorigenic HPV. The seven isoforms bound variant and mutated phospho-motifs of E6 and unrelated protein RSK1 with different affinities, albeit following an ordered ranking profile with conserved relative KD ratios. Remarkably, 14-3-3 isoforms obey the same hierarchy when binding to most of their established targets, nicely supported by a recent proteome-wide human complexome map. This knowledge allows predicting the proportions of 14-3-3 isoforms engaged with phosphoproteins in various tissues. Notwithstanding their individual functions, cellular concentrations of 14-3-3 may be collectively adjusted to buffer the strongest phosphorylation outbursts, explaining their expression variations in different tissues and tumors.