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

The novel multidomain protein, cpSRP43, is a unique subunit of the post-translational chloroplast signal recognition particle (cpSRP) targeting pathway in higher plants. The cpSRP pathway is responsible for targeting and insertion of light-harvesting chlorophyll a/b binding proteins (LHCPs) to the thylakoid membrane. Upon emergence into the stroma, LHCPs form a soluble transit complex with the cpSRP heterodimer, which is composed of cpSRP43 and cpSRP54. cpSRP43 is irreplaceable as a chaperone to LHCPs in their translocation to the thylakoid membrane and remarkable in its ability to dissolve aggregates of LHCPs without the need for external energy input. In previous studies, cpSRP43 has demonstrated significant flexibility and interdomain dynamics. In this study, we explore the structural stability and flexibility of cpSRP43 using a combination of computational and experimental techniques and 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 variables are used along with biophysical experimentation to explain the basis of the flexibility and stability of cpSRP43, showing that the free and cpSRP54-bound cpSRP43 has substantially different conformations and conformational dynamics.