In general, protein targeting involves a signal recognition particle (SRP), an SRP receptor and a translocase. While co-translational targeting is often mediated in the cell by a ribosome, post-translational targeting can be accomplished without. An important example is that of targeting the light-harvesting chlorophyll-binding protein (LHCP) to the thylakoid membrane in chloroplasts. Solar energy conversion by photosynthesis relies on the efficient targeting and assembly of such pigment-binding proteins for harvesting light. In chloroplasts, a multi-domain subunit of the SRP, cpSRP43, has been proposed to take on the role of coordinating the sequence of targeting events in the absence of the ribosome, highlighted by the fact that the process has been fully reconstituted <i>in vitro.</i> Such a process may offer opportunities for biotechnological developments in energy conversion.<br/><br/>cpSRP43 is a multidomain protein that forms a heterodimer with the conserved GTPase, cpSRP54, which subsequently interacts with the LHCP substrate to form a soluble targeting complex. Using single-molecule Förster Resonance Energy Transfer (smFRET), molecular dynamics (MD) simulations and isothermal titration calorimetry (ITC), we have revealed significant inter-domain dynamics in cpSRP43. Upon binding to cpSRP43, we found that the affinity of the L18 recognition site of LHCP to cpSRP43 increased when cpSRP43 was complexed to a cpSRP54 peptide (corresponding to the cpSRP43 binding site on cpSRP54). These results suggest that nature has adopted/evolved a strategy for improving affinity for protein-targeting substrates by using biomolecular interactions to regulate inter-domain dynamics. These results may have implications in biotechnology allowing us to use such lessons from biology for targeting and assembling renewable biomolecular-based components in applications such as renewable energy.

Promoting binding of protein-targeting substrates by regulating interdomain dynamics within a signal recognition particle: Implications for biotechnology

249th National Meeting (2015)

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