Regulation of Structural Dynamics within a Signal Recognition Particle Promotes Binding of Protein Targeting Substrates

by Gao, Feng; Kight, Alicia D.; Henderson, Rory; Jayanthi, Srinivas; Patel, Parth; Murchison, Marissa; Sharma, Priyanka; Goforth, Robyn L.; Kumar, Thallapuranam Krishnaswamy Suresh; Henry, Ralph L.; Heyes, Colin D.

Background: Targeting of proteins requires a signal recognition particle (SRP) and multiple protein interactions. Results: We observed a decrease in the structural dynamics of cpSRP43 and an increase in substrate affinity upon its binding to cpSRP54. Conclusion: Changes in domain dynamics induced by cpSRP subunit interactions mediate substrate affinity. Significance: Relating structure and dynamics of SRP proteins allows for a better understanding of vectorial targeting within cells. Protein targeting is critical in all living organisms and involves a signal recognition particle (SRP), an SRP receptor, and a translocase. In co-translational targeting, interactions among these proteins are mediated by the ribosome. In chloroplasts, the light-harvesting chlorophyll-binding protein (LHCP) in the thylakoid membrane is targeted post-translationally without a ribosome. A multidomain chloroplast-specific subunit of the SRP, cpSRP43, is proposed to take on the role of coordinating the sequence of targeting events. Here, we demonstrate that cpSRP43 exhibits significant interdomain dynamics that are reduced upon binding its SRP binding partner, cpSRP54. We showed that the affinity of cpSRP43 for the binding motif of LHCP (L18) increases when cpSRP43 is complexed to the binding motif of cpSRP54 (cpSRP54(pep)). These results support the conclusion that substrate binding to the chloroplast SRP is modulated by protein structural dynamics in which a major role of cpSRP54 is to improve substrate binding efficiency to the cpSRP.

Journal of Biological Chemistry
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1083-351X; 0021-9258