D1 protein processing and Mn cluster assembly in light of the emerging Photosystem II structure

The formation of the (Mn-4-Ca) center of the water oxidation complex (WOC) of Photosystem II (PSII) occurs via a complex post-translational assembly pathway involving the proteolytic processing of the D1 protein precursor and the light-driven assembly, termed photoactivation, of the (Mn-4-Ca). Photoactivation consists of a sequence of light-activated steps, involving Mn oxidation, separated by 'dark' molecular rearrangements, which are necessary for creating the coordination environment for the incoming metals. Initially, only one high affinity Mn binding site exists in the apo-WOC, whereas the remaining metal binding sites are created during the photoactivation process. Although much progress has been made in defining these processes, the molecular details remain obscure. Recently, X-ray crystallographic analyses have begun to reveal the molecular structure of PSII raising the possibility of formulating tentative structure-based mechanisms for the proteolytic processing and (Mn-4-Ca) photoassembly events. Here an attempt is made to review and assemble the current kinetic and structural data to begin to develop a plausible molecular model for the assembly of the (Mn-4-Ca) with particular emphasis placed on the proteolytic processing of the D1 protein precursor and the nature of 'dark' rearrangements occurring on the assembly of the (Mn-4-Ca).