Structural and functional aspects of the MSP (PsbO) and study of its differences in thermophilic versus mesophilic organisms

Abstract

The Manganese Stabilizing Protein (MSP) of Photosystem II (PSII) is a so-called extrinsic subunit, which reversibly associates with the other membrane-bound PSII subunits. The MSP is essential for maximum rates of O(2) production under physiological conditions as stabilizes the catalytic [Mn(4)Ca] cluster, which is the site of water oxidation. The function of the MSP subunit in the PSII complex has been extensively studied in higher plants, and the structure of non-PSII associated MSP has been studied by low-resolution biophysical techniques. Recently, crystal structures of PSII from the thermophilic cyanobacterium Thermosynechococcus elongatus have resolved the MSP subunit in its PSII-associated state. However, neither any crystal structure is available yet for MSP from mesophilic organisms, higher plants or algae nor has the non-PSII associated form of MSP been crystallized. This article reviews the current understanding of the structure, dynamics, and function of MSP, with a particular focus on properties of the MSP from T. elongatus that may be attributable to the thermophilic ecology of this organism rather than being general features of MSP.

The evolution of Photosystem II: insights into the past and future

“…The number and identity of extrinsic subunits varies depending on the species, but always includes the MSP, the presence of which is essential for optimal functioning of water splitting under physiological conditions. The MSP is a b-barrel with a large number of flexible loop regions, and spinach MSP has been characterized as a molten globular or natively unfolded structure (Williamson 2008 and references therein).…”

Function of Extrinsic Proteins in Stabilization of the Photosynthetic Oxygen-Evolving Complex

“…These proteins play a key role in maintaining oxygen-evolving activity at physiological rates [Roose et al, 2007, Williamson, 2008. PsbO independently associates with the PSII core , Miyao & Murata, 1989, and with PsbP through electrostatic interactions with PsbO , Tohri et al, 2004.…”

“…The release of PsbO induces release of Mn ions from the cluster, resulting in the loss of O 2 -evolving activity. The PsbO protein is common in every oxygenic phototroph but in varying proportions: one PsbO per PSII in cyanobacteria and two PsbO per PSII in higher plants [Williamson, 2008, Xu & Bricker, 1992. Highresolution X-ray crystallographs of the PsbO protein associated with the PSII core are available for Thermosynechococcus elongatus [Ferreira et al, 2004 and Thermosynechococcus vulcanus [Kawakami et al, 2009, Umena et al, 2011, in which PsbO is comprised of a -barrel core with an extended -helix domain.…”

“…Findings to date strongly indicate another significant role for PsbO: it is thought to modulate Ca 2+ and Cl -requirements for O 2 evolution [Seidler, 1996, Williamson, 2008. However, the Ca 2+ -binding properties of PsbO proteins are somewhat different between higher plants and cyanobacteria.…”

“…PsbP and PsbQ are indigenous to plant PSII and have been proposed as the functional equivalents of PsbV and PsbU in bacterial PSII, having replaced them during the course of evolution from ancestral cyanobacteria to higher plants. These proteins play significant roles in the regulation and stabilization of the photosynthetic water oxidation [Roose et al, 2007, Seidler, 1996, Williamson, 2008 although the details of their function(s) are still a matter of debate. In this chapter, we describe the structural-functional roles of extrinsic proteins in the plant PSII.…”