The extrinsic proteins of an oxygen-evolving complex in marine diatom Cylindrotheca fusiformis

Diatom Cylindrotheca fusiformis's oxygen-evolving complex (OEC) exhibited much lower 2,6-dichlorophenylindophenol (DCIP) photoreduction activity (the electron transfer rate during O-2 evolution) when compared to that of spinach. The protein composition of the OEC in C. fusiformis diatom thylakoids consist of only two extrinsic proteins with apparent molecular masses of 33 kDa and 15 kDa after Tris or NaCl washing. The 33 kDa protein was mainly responsible for O-2 evolving activity and for about 60% of the DCIP photoreduction activity, whereas the 15 kDa protein's photoreduction activity was approximately 40%. Beside the two extrinsic proteins, we found that Ca2+, Cl- and Mn ions also participate in the O-2 evolution of C. fusiformis. The concentrations of Ca2+ and Cl- required for O-2 evolution in diatom thylakoids were higher than those for spinach, suggesting that the binding affinity of Ca2+ and Cl- in diatom OEC is lower than that of spinach. The 33 kDa and 15 kDa proteins may coincidently function in Ca2+ trapping due to a 23% diminishment in content when diatom thylakoids were depleted of both proteins. These two proteins were also involved in the binding of Mn ions. Analysis of amino acid occurrence frequencies confirmed that both 33 kDa and 15 kDa proteins were extrinsic.