Moonlighting chaperone-like activity of the universal regulatory 14-3-3 proteins
The ubiquitous eukaryotic 14-3-3 proteins coordinate multiple cellular processes due to their well-known regulatory function, which is based on specific recognition of phosphorylated motifs in their partners. In this context, 14-3-3 proteins have been called ‘chaperones’. Although in the classical meaning this is not fully correct, recent studies have revealed that they can indeed be an integral part of the protein quality control system, as they (a) display ATP-independent anti-aggregation (‘holdase’) activity, similar to that of the unrelated small heat shock proteins, (b) assist in clearing misfolded proteins by directing them to proteasomes or aggresomes, (c) cooperate with classical chaperones for substrate refolding, and also (d) are associated with neurodegenerative disorders by affecting aggregation of tau, prion protein, α-synuclein, huntingtin, etc. Importantly, these activities are usually independent of substrate phosphorylation and therefore should be considered as distinct, ‘moonlighting’ functions of 14-3-3 proteins that mimic and complement the functions of dedicated molecular chaperones. Although the precise mechanism of this activity is still unknown, it has been shown that it is not dependent on the unstructured C-terminal region or the amphipathic phosphopeptide-binding groove. However, since disassembly of 14-3-3 dimers significantly increases their chaperone-like activity, the dimer interface, located in the N terminus, possessing a high disorder propensity and pronounced hydrophobicity, is likely to be involved. Various factors affecting the oligomeric status of 14-3-3 proteins can thus regulate the balance between regulatory phosphomotif binding and genuine chaperone-like activity. Understanding the latter mode of 14-3-3 functioning is fundamental to defining the underlying molecular mechanisms for a range of human disorders.
The ubiquitous and mighty 14-3-3 proteins regulate a plethora of cellular processes via coordinated binding to hundreds of specifically phosphorylated partners. Recent evidence that 14-3-3 proteins interfere with protein aggregate formation suggests they can also display a distinct, usually phosphorylation-independent, chaperone-like activity, which we review here as their ‘moonlighting’ function, which is of particular relevance in the context of neurodegenerative disorders.