Location of photosystem I and photosystem II reaction centers in different thylakoid regions of stacked chloroplasts

Abstract

Stacked chloroplast thylakoids are structurally differentiated into the appressed regions of the grana (partitions), and the non-appressed regions of grana end membranes, margins and stroma lamellae. This structural differentiation is also accompanied by differences in function (cf. [1-3]). In early fractionation studies small stroma lamellae vesicles highly enriched in photosystem (PS)I were separated from rapidly sedimenting grana containing both PSI and PS2 [4]. This suggested that the grana were the site for linear electron transport by PS 1 and PS2 in close physical association while stroma lamellae was mainly involved in PSI-mediated cyclic electron flow. This view of thylakoid organization has been challenged by the availability of a fraction enriched in inside-out vesicles shown to originate from the appressed grana partitions [5-8]. Analyses of this fraction with respect to photochemical activities [8] and content of chlorophyll(Chl)-proteins [9] revealed a pronounced PSI depletion. Based upon this, it was postulated that PS 1 is mainly excluded from the appressed grana regions into the non-appressed regions thereby separated from most of PS2 which is concentrated in the appressed regions [2,9,10]. This model implies linear electron transport between spatially separated photosystems involving lateral shuttling of electron carriers between the different thylakoid regions. Here the amount of PSI and PS2 reactions centres in the various thylakoid subfractions were directly determined by measuring the light-induced absorbance changes of P700 and Q, respectively. The results present experimental evidence for the suggested, extreme lateral heterogeneity in the location of the two photosystems, particularly by demonstrating virtually no P700 in the appressed grana partitions.