Thermal unfolding of the manganese stabilizing 33 kD protein photosystem II: implications for a molten globular state

Abstract

Manganese stabilizing 33 kDa protein is a constituent of PS II in all oxygenic photosynthetic organisms from cyanobacteria to higher plants and plays an important role in maintaining the stability and efficient turnover of the tetranucler manganese cluster of the water oxidizing complex. We examined the thermal unfolding of the MSP by using differential scanning calorimetry. In low ionic strength buffer at pH 6.5, thermal unfolding of the MSP was resolved as an extended peak (35-130¡æ) . The deconvolution analysis clearly established the presence of no two-state transition with midpoint Tm = 100.8 ¡æ, however, it appears that irreversible thermal denaturation of MSP occurred in the range of 120°C. This is an unusually high value and probably reflects the role of the MSP as an external structural protein in photosystem II. Upon addition of Ca2+ ions, the Tm of the MSP was about 2°C lower than that of MSP in the absence Ca2+. These results demonstrated that MSP undergoes a low-cooperativity non-two-state thermal unfolding. This is first evidence support the hypothesis recently proposed that the MSP in solution maybe attain a molten globular state, which is defined as a compact folding intermediate with a near-native secondary structure but lax tertiary structure. Our results suggest that MSP may perform its major physiological function via the molten globular state in vivo.