Understanding the Role of Ankyrin Domain of the 43-Kda Subunit of the Chloroplast Signal Recognition Particle in Protein Targeting

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Light-harvesting chlorophyll-binding proteins (LHCPs) play an important role in photosynthesis. Photosynthesis occurs in the chloroplast of plant cells, while LHCPs are synthesized in the cytoplasm. One of the many paths utilized to target proteins to the thylakoid membrane of chloroplasts is the signal recognition particle (cpSRP) pathway. LHCPs form a transit complex with cpSRP43 and cpSRP54, the two subunits of the chloroplast signal recognition particle, in the cytoplasm before being targeted and integrated into the thylakoid membrane of the chloroplast. The transit complex binds to Albino 3 (Alb3), a protein in the thylakoid membrane. Of specific interest is the Ankyrin (Ank) domain of cpSRP43. Ank domains are found in a wide variety of proteins and they are found to be functionally important. In the LHCP-cpSRP complex, part of the Ank domain is responsible for binding LHCP to the complex. Recent studies have shown that the Ank domain of cpSRP43 plays a significant role in binding to Alb3 and in LHCP integration. The present study aims to understand the interactions between the c-terminal end of Alb3 (cAlb3) and the Ankyrin domain (Ank) of cpSRP43. The proposed research plans to characterize the conformation and stability of Ank and Alb3 using circular dichroism and fluorescence. The binding affinities of Ank and cAlb3 will be examined using isothermal titration calorimetry. Finally, the binding interface between Ank and cAlb3 will be mapped using 1H-15N chemical shift perturbation. The results of this study are expected to achieve a greater understanding of LHCP integration to the thylakoid membrane and protein transport in general.