The CorA family: Structure and function revisited

The CorA family is a group of ion transporters that mediate transport of divalent metal ions across biological membranes. Metal ions are essential elements in most cellular processes and hence the concentrations of ions in cells and organelles must be kept at appropriate levels. Impairment of these systems is implied in a number of pathological conditions. CorA proteins are abundant among the prokaryotic organisms but homologues are present in both human and yeast. The activity of CorA proteins has generally been associated with the transport of magnesium ions but the members of the CorA family can also transport other ions such as cobalt and nickel. The structure of the CorA from Thermotoga maritima, which also was the first structure of a divalent cation transporter determined, has opened the possibilities for understanding the mechanisms behind the ion transport and also corrected a number of assumptions that have been made in the past.

Functional characterization of the PS II-LHC II supercomplex isolated by a direct method from spinach thylakoid membranes

Recently, a novel procedure to isolate a highly pure and active Photosystem II preparation directly from thylakoid membranes, referred to as PS II-LHC II supercomplex, was reported [Eshaghi et al. (1999) FEBS Lett 446: 23-26]. In addition to the reaction center core proteins, the supercomplex contains all the extrinsic proteins of the oxygen evolving complex and a set of chlorophyll a/b binding proteins. In this paper, the functional properties of this isolated supercomplex are further characterized by using EPR spectroscopy, thermoluminescence, fluorescence relaxation kinetics and flash induced oxygen yield measurements. The PS II-LHC II supercomplex contains, in addition to Q(A) and Q(B), a small pool of plastoquinone (PQ). Although the isolated complex is no longer membrane bound, it has preserved functional characteristics of a well defined PS II preparation with the exception of some modification of Q(B) sites.

Photoassembly of the manganese cluster and oxygen evolution from monomeric and dimeric CP47 reaction center photosystem II complexes

Isolated subcomplexes of photosystem II from spinach (CP47RC), composed of D1, D2, cytochrome b(559), CP47, and a number of hydrophobic small subunits but devoid of CP43 and the extrinsic proteins of the oxygen-evolving complex, were shown to reconstitute the Mn(4)Ca(1)Cl(x) cluster of the water-splitting system and to evolve oxygen. The photoactivation process in CP47RC dimers proceeds by the same two-step mechanism as observed in PSII membranes and exhibits the same stoichiometry for Mn(2+), but with a 10-fold lower affinity for Ca(2+) and an increased susceptibility to photodamage. After the lower Ca(2+) affinity and the 10-fold smaller absorption cross-section for photons in CP47 dimers is taken into account, the intrinsic rate constant for the rate-limiting calcium-dependent dark step is indistinguishable for the two systems. The monomeric form of CP47RC also showed capacity to photoactivate and catalyze water oxidation, but with lower activity than the dimeric form and increased susceptibility to photodamage. After optimization of the various parameters affecting the photoactivation process in dimeric CP47RC subcores, 18% of the complexes were functionally reconstituted and the quantum efficiency for oxygen production by reactivated centers approached 96% of that observed for reconstituted photosystem II-enriched membranes.

Isolation of a highly active PSII-LHCII supercomplex from thylakoid membranes by a direct method

We have developed a simple and novel method to isolate a highly pure and active photosystem (PS) II complex, directly from thylakoid membranes. This complex is a discrete particle and contains all the proteins of the oxygen evolving complex and a set of chlorophyll alb binding proteins. The intactness of both the donor side and the acceptor side has resulted in a very high oxygen evolution activity and therefore offers a superior experimental system to that of PSII enriched membrane fragments in which there is heterogeneity in activities and biochemical composition.