Chimeric 14-3-3 proteins for unraveling interactions with intrinsically disordered partners

In eukaryotes, several "hub" proteins integrate signals from different interacting partners that bind through intrinsically disordered regions. The 14-3-3 protein hub, which plays wide-ranging roles in cellular processes, has been linked to numerous human disorders and is a promising target for therapeutic intervention. Partner proteins usually bind via insertion of a phosphopeptide into an amphipathic groove of 14-3-3. Structural plasticity in the groove generates promiscuity allowing accommodation of hundreds of different partners. So far, accurate structural information has been derived for only a few 14-3-3 complexes with phosphopeptide-containing proteins and a variety of complexes with short synthetic peptides. To further advance structural studies, here we propose a novel approach based on fusing 14-3-3 proteins with the target partner peptide sequences. Such chimeric proteins are easy to design, express, purify and crystallize. Peptide attachment to the C terminus of 14-3-3 via an optimal linker allows its phosphorylation by protein kinase A during bacterial co-expression and subsequent binding at the amphipathic groove. Crystal structures of 14-3-3 chimeras with three different peptides provide detailed structural information on peptide-14-3-3 interactions. This simple but powerful approach, employing chimeric proteins, can reinvigorate studies of 14-3-3/phosphoprotein assemblies, including those with challenging low-affinity partners, and may facilitate the design of novel biosensors

Hybrid coupling of R-phycoerythrin and the orange carotenoid protein supports the FRET-based mechanism of cyanobacterial photoprotection

To regulate the effectiveness of photosynthesis and photoprotection cyanobacteria utilize a system consisting of only few components. Photoactivation of the orange carotenoid protein (OCP) enables its interaction with a specific, yet controversial site in the core of the light-harvesting antenna, the phycobilisome (PBS). The resulting delivery of a quenching carotenoid molecule to the antenna pigments leads to thermal dissipation of the excitation energy absorbed by the latter, and, consequently, to depression of the photosynthetic activity. The nature of the OCP-induced PBS fluorescence quenching mechanism remains debatable, however, specific protein-protein interactions between PBS and photoactivated OCP should provide a unique environment for interactions between the excitation energy donor and acceptor. Here we questioned whether the F&euro;orster theory of resonance energy transfer can explain PBS quenching by OCP even at their very small spectral overlap and whether in model systems, the absence of specific protein-protein interactions of OCP with a donor of energy can be compensated by a better spectral overlap. Hybridization of algal R-phycoerythrin with cyanobacterial OCP by chemical crosslinking results in a significant decrease of R-phycoerythrin fluorescence lifetime, irrespective of the OCP photoactivation status. Supported by structural considerations, this indicates that FRET may be the essence of cyanobacterial photoprotection mechanism. &nbsp;</p