On the Localization of the Synthesis of Photosynthetic Proteins

Abstract

Thylakoid membrane biogenesis requires polypeptides subunits of the photosynthesis complexes, their cofactors and pigments, and the galactolipids of the membrane bilayer. Although the pathways that synthesize and assemble these components are known or being elucidated, much less is known about the cytological organization of these pathways. Results of fluorescence confocal microscopy support the existence of a novel chloroplast compartment that is specialized in photosystem II biogenesis and possibly the synthesis of other components of thylakoid membranes. We are substantiating these findings with results of cellular subfractionation experiments, which reveal a that a minor class of chloroplast membranes has markers of a thylakoid biogenesis compartment and are distinct from the membranes of thylakoids or the chloroplast envelope (unpublished data). The characterization of these “thylakoid biogenesis membranes” could help reconcile the contradictions in the available evidence regarding where thylakoid membrane components are synthesized and how they are transported. For example, not much is known about how the thylakoid proteins encoded by the chloroplast genome are targeted for thylakoid membrane biogenesis. Our results reveal that mRNA-based localization of protein synthesis localizes PS II subunits and may be a prevalent targeting mechanism in chloroplasts.