Chloroplast SRP43 Subunit Prevents Aggregation of Proteins

Abstract

Integration of light-harvesting chlorophyll binding proteins into the thylakoid membrane requires a specific chaperone, being the cpSRP43 subunit, of the signal recognition particle pathway in chloroplasts. cpSRP43, unique to the chloroplast, is responsible for transport of LHCPs through the stroma as well as assisting in the correct folding, assembly and disaggregation of these proteins for the acquisition of light energy. cpSRP43 is a highly flexible, multidomain protein capable of binding distinct partners in the cpSRP pathway. cpSRP43 is an irreplaceable component, necessary for the accurate and successful integration of LHCPs. It can act as a disaggregase without any input of external energy. Its action is based on the ability to associate with variable regions of different proteins owing to the domains and flexibility within its distinctive structure. Understanding the unique capabilities of cpSRP43 in the chloroplast begs the question of its usefulness outside of the plant cell, as well as its yet unknown roles still within the plant cell. Although the capabilities of cpSRP43 as a hub protein, adept to binding many unknown partners, has been alluded to in other works, it has yet to be thoroughly investigated. In this study we discover that cpSRP43 can act as a generic chaperone for proteins other than LHCP/not native to the chloroplast. The high thermal stability of cpSRP43 has been demonstrated in the previous chapter by its ability to retain its secondary structure as well as withstand aggregation upon heating and cooling cycles as confirmed by absorbance, intrinsic tryptophan fluorescence and far UV circular dichroism spectroscopy. This property gives cpSRP43 the basis to act as a generic chaperone and provide protection like that of typical heat shock proteins. Carbonic anhydrase, Concanavalin A and hFGF1 (acidic human fibroblast growth factor), were selected as candidates for chaperoning activity by cpSRP43. In all three cases, heat-induced aggregation of the candidate protein was either eliminated or significantly reduced in the presence of cpSRP43. In the case of hFGF1, the bioactivity was preserved after heat-treatment in the presence of cpSRP43. We have proposed a mechanism by which cpSRP43 is able to execute this action however further investigation is warranted to determine the exact mechanism(s) which may vary dependent on the target protein.