Download Auxiliary proteins of photosystem II: tuning the enzyme for optimal

Auxiliary proteins of photosystem II: tuning the enzyme for optimal activity
Julian J. Eaton-Rye
Department of Biochemistry, Otago University, P. O. Box 56, Dunedin
Abstract
The core of Photosystem II (PS II) is made up of two reaction center proteins, D1 (PsbA) and D2
(PsbD) and two chlorophyll a-binding antenna proteins, CP47 (PsbB) and CP43 (PsbC). These
proteins have homologues in anoxygenic photosynthetic bacterial reaction centers; however, PS II has
an increased complement of polypeptides. These proteins include ~12 low-molecular-weight (LMW)
hydrophobic polypeptides that form a ring around the core and a “cap” of 3 or more hydrophilic
polypeptides that cover the oxygen-evolving center (OEC). These auxiliary PS II proteins were
acquired prior to endosymbiotic uptake of cyanobacteria. The crystal structure of cyanobacterial PS II
has revealed that the cap of the OEC is composed of three polypeptides: PsbO, PsbU and PsbV, and
two large loops from CP43 and CP47. In green algae and plants, PsbU and PsbV are absent but two
different proteins, PsbP and PsbQ, are present. However, biochemical and genomic studies have now
established that there are PsbP and PsbQ homologues in all photosynthetic lineages: and in
cyanobacteria the corresponding homologues, CyanoP and CyanoQ, are lipidated at their N-termini.
Successful water splitting by PS II carries with it the metabolic cost of light-induced photodamage
(chiefly to D1) and the resulting need for a repair cycle to support ongoing O2-evolving activity. It is
likely that a major role of the auxiliary proteins is to assist in water splitting and PS II repair. We have
investigated the role of the LMW PsbT protein: removal of this subunit increased susceptibility to
photodamage and accelerated the rate of incorporation of replacement D1. Moreover, in this mutant,
electron transfer between QA and QB, the primary and secondary quinone electron acceptors of PS II,
was slowed and the PSII-specific electron acceptor 2,5 dimethyl-p-benzoquinone blocked QAoxidation. These effects could be prevented, and in some cases reversed, by the addition of HCO3-, a
PS II-specific cofactor that binds to the non-heme iron between QA and QB. In addition, we have
obtained the X-ray-derived structure of CyanoQ and the NMR structures of CyanoP and a third
cyanobacterial lipoprotein, Psb27 (Psb27 is found in all photosynthetic lineages and is involved in the
PS II repair cycle). Structural features of these polypeptides will be presented together with docking
models that place these proteins adjacent to channels in the OEC cap that may be required to allow
substrate H2O or cofactors to enter, and O2 and protons to exit, to and from the Mn4O5Ca active site of
the OEC.
Biography
Associate Professor Julian Eaton-Rye completed a BSc Honors degree in Botany from the University
of Manchester, UK in 1981 and obtained his PhD (Plant Physiology) in 1987 at the University of
Illinois at Urbana-Champaign. After postdoctoral research at the National Institute of Basic Biology,
Okazaki, Japan, Arizona State University, and Brookhaven National Laboratory, USA, he moved to
Otago University in 1994. Throughout his career he has concentrated on the biogenesis, structure and
function of photosystem II: the light-driven water-splitting enzyme of photosynthesis. Many studies in
his laboratory have utilised the cyanobacterium Synechocystis sp. PCC 6803 as a model organism and
combine physiological, biochemical and molecular genetic approaches to understanding the molecular
details of how the protein framework of the photosystem supports the sustained oxygen-evolving
activity of this enzyme across a wide range of environmental conditions. Currently the lab is
concentrating on the roles of a number of polypetide subunits that are essential in the “life-cycle” of
this enzyme but are not present in the X-ray-derived structures obtained for the mature complex.