N‐Bromosuccinimide modification of tryptophan 241 at the C‐terminus of the manganese stabilizing protein of plant photosystem II influences its structure and function

Abstract

Life on earth depends upon the ability of oxygenic photosynthesis to oxidize water to molecular oxygen. This process is catalyzed by water–plastoquinone oxido‐reductase complex. In addition to the photosystem II (PSII) reaction core, it includes a manganese stabilizing protein (MSP) that plays an important regulatory role in the process in plants and algae. Tryptophan 241, located at the carboxyl‐terminus of the MSP, is its sole tryptophan. Modification of MSP by N‐bromosuccinimide (NBS) was carried out to explore the role of Trp241 in maintaining its structure and function. Data and arguments are presented to show that it is Trp241, not other tyrosines in MSP, that is involved in the modification and changes observed in this study. Further, the pH‐dependence of the modification and the comparison of features of fluorescence spectra of MSP suggested that Trp241 is buried in the hydrophobic interior of the protein. Hydropathy analysis revealed that Trp241 is located in the middle of the hydrophobic region at the C‐terminus of MSP. Circular dichroism spectroscopy showed that NBS modification of Trp241 dramatically modified the protein structure. The affinity of MSP to PSII decreased greatly after the modification of Trp241, and no oxygen‐evolving activity was recovered after its reconstitution. This study provides a novel demonstration that Trp241 at the C‐terminus hydrophobic region of the MSP is critical for maintaining appropriate structure and function of MSP.