cpSRP43 Is Both Highly Flexible and Stable: Structural Insights Using a Combined Experimental and Computational Approach

The novel multidomain protein, cpSRP43, is a unique subunitofthe post-translational chloroplast signal recognition particle (cpSRP)targeting pathway in higher plants. The cpSRP pathway is responsiblefor targeting and insertion of light-harvesting chlorophyll a/b bindingproteins (LHCPs) to the thylakoid membrane. Upon emergence into thestroma, LHCPs form a soluble transit complex with the cpSRP heterodimer,which is composed of cpSRP43 and cpSRP54. cpSRP43 is irreplaceableas a chaperone to LHCPs in their translocation to the thylakoid membraneand remarkable in its ability to dissolve aggregates of LHCPs withoutthe need for external energy input. In previous studies, cpSRP43 hasdemonstrated significant flexibility and interdomain dynamics. Inthis study, we explore the structural stability and flexibility ofcpSRP43 using a combination of computational and experimental techniquesand find that this protein is concurrently highly stable and flexible.In addition to microsecond-level unbiased molecular dynamics (MD),biased MD simulations based on system-specific collective variablesare used along with biophysical experimentation to explain the basisof the flexibility and stability of cpSRP43, showing that the freeand cpSRP54-bound cpSRP43 has substantially different conformationsand conformational dynamics.