C-Terminal M4 Peptide Fragment of Albino 3 Interaction with cpSRP-43

Abstract

The protein complexes responsible for insertion of light harvesting chlorophyll binding protein into the thylakoid membrane of the chloroplast are the concern of this investigation. Critical for this insertion, the membrane bound protein Albino 3 has been shown to bind a cpSRP-FTSY-LHCP complex. The following study implicates the stromal, c-terminus of Albino 3 as an imperative binding interaction in this process. Isothermal titration calorimetry and 1H-15N HSQC were used to understand the binding of a 16 residue segment from this region of Albino 3 known as M4. The data show that this segment binds with 14uM affinity and that electrostatic interactions with E327 and E356 on the 43kDa subunit of cpSRP are essential for this event. Importantly, significant binding is not observed when two domains of cpSRP-43, CD2 and CD3, are separated. Far UV and intrinsic fluorescence experiments also show that, upon M4 binding, moderate conformational changes in CD2-CD3 occur.