Review Update: 14-3-3 Proteins: A Number of Functions for a Numbered Protein

Dave Bridges¹ and Greg B. G. Moorhead^2*

¹Life Sciences Institute, University of Michigan, 210 Washtenaw Avenue, Ann Arbor, MI 48109, USA.
²Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4.

Since this Review was first published, there have been a number of advances in the 14-3-3 field. This Reader’s Guide highlights some of the new material covered in the updated Review.

Structure and Function

Crystallization of the nonsense-mediated decay regulatory protein, SMG7 (suppressor with morphogenetic effects on genitalia 7), revealed structural resemblance of one of its TPR (tetratricopeptide repeat) domains to one monomer of a 14-3-3 protein. Further research showed that this region binds peptides in a phosphorylation-dependent manner. This suggests that there may be other structural homologs of 14-3-3 proteins. Furthermore, it appears that this can occur with very limited (<10%) sequence homology (1).

The crystallization of 14-3-3 revealed a structural basis for this isoform’s preferential homodimerization, as well as for its specific selectivity for target binding proteins. Partner specificity seems to be the result of amino acid differences outside of the phosphopeptide binding groove (2).

Regulation of 14-3-3 Proteins

Evidence is mounting that 14-3-3 is unique among 14-3-3 isoforms. This isoform is evolutionarily distant from the other 14-3-3 proteins and appears to be the result of a retrotransposon insertion event. Moreover, this isoform preferentially undergoes homo- but not heterodimerization and exhibits a binding partner specificity distinct from that of the other isoforms (2).

Phosphorylation of 14-3-3 on serine 233 results in decreased target binding. Phosphorylation of a residue in this region of the protein, which normally occupies the phosphopeptide binding groove (3), may enhance the affinity of the carboxy terminus for the phosphopeptide binding groove. The region surrounding the Ser²³³ site is a suboptimal binding motif, but it may competitively inhibit target binding once phosphorylated, and this inhibition may be overcome by high-affinity targets (3, 4).

The 14-3-3 Interactome

Since publication of this review, three more 14-3-3 interactome studies have been published. All are notable not only for the proteins identified, but also because the proteins identified in these various studies differ greatly (5–7).

It is estimated that there is only approximately 25% overlap between the interactomes identified in these studies. The starting material differed in several of these studies, as did the 14-3-3 isoform used, making it difficult to compare these results. One would predict that the immobilized affinity matrix employed by one group may be biased toward low-affinity (rapidly exchanging) targets, whereas the single isoform approach used by the other groups may skew the results toward isoform-specific interactors.

The most recent study, in which 14-3-3 was used as the target, showed only 17% overlap with the binding proteins identified by 14-3-3, in spite of similar methodology and starting material (7). As more of these studies are done, more isoform- and tissue-specific interactors will be found. This highlights the importance of determining which method of identifying 14-3-3 binding proteins is most accurate from a physiological standpoint. The recent work suggesting isoform-specific interactions, combined with the potential heterodimerization complexity of 14-3-3 dimers, will complicate proteomic studies even further.

References