Abstract

The 14-3-3 proteins constitute a family of conserved proteins present in all eukaryotic organisms so far investigated, including seven isotypes in human cells. They occur as homo- or heterodimers. They are involved in important cellular processes such as signal transduction, cell-cycle control, apoptosis, stress response and malignant transformation. More than one hundred different binding partners for these proteins have been reported so far. 14-3-3 proteins were the first signaling molecules to be identified as discrete phosphoserine/threonine binding modules, although cases of phosphorylation-independent interaction have been reported. While many details of the biochemical and cellular functions of 14-3-3 proteins remain to be elucidated, they are known to act as adaptor molecules to mediate protein-protein interactions, the subcellular localisation and to regulate enzyme activities. Though 14-3-3 proteins perform different functions for different ligands, general mechanisms of 14-3-3 action include changes in activity of bound ligands, altered association of bound ligands with other cellular components, and changes in intracellular localization of 14-3-3-bound cargo.

The classical mode 1 and mode 2 binding motifs are compatible with phosphorylation by e.g. PKA, PKB and other kinases with a positive charge preference and it is likely that these are the typical kinases phosphorylating 14-3-3-binding sites. On the other hand the C-terminal peptide from plant H+ ATPases - QSYpTV* - lacks a positive charge and will be phosphorylated by another class of kinase. This does raise the question of how often non-canonical 14-3-3 binding peptides will lack a positive residue preceding the phosphorylation site.

14-3-3 proteins are targeted by certain fungal toxins and bacterial virulence factors. Fusicoccin toxin stabilises an H+ ATPase phosphopeptide interaction with 14-3-3 (Wuertele et al., 2003; PMID:12606564). The non-phosphorylated Pseudomonas exoS peptide LLDALDL binds in the reverse orientation to the phosphorylated peptides and it is possible that there are cellular equivalents of this binding mode (Ottmann et al., 2007; PMID:17235285). It is thought that all non-phosphorylated peptides will bind in the reverse orientation.