The Vesicle-inducing Protein 1 from Synechocystis sp. PCC 6803 Organizes into Diverse Higher-Ordered Ring Structures

Abstract

The vesicle-inducing protein in plastids 1 (Vipp1) was found to be involved in thylakoid membrane formation in chloroplasts and cyanobacteria. In contrast to chloroplasts, it has been suggested that in cyanobacteria the protein is only tightly associated with the cytoplasmic membrane. In the present study we analyze and describe the subcellular localization and the oligomeric organization of Vipp1 from the cyanobacterium Synechocystis PCC 6803. Vipp1 forms stable dimers and higher-ordered oligomers in the cytoplasm as well as at both the cytoplasmic and thylakoid membrane. Vipp1 oligomers are organized in ring structures with a variable diameter of 25-33 nm and corresponding calculated molecular masses of ϳ1.6 -2.2 MDa. Six different types of rings were found with an unusual 12-17-fold symmetrical conformation. The simultaneous existence of multiple types of rings is very unusual and suggests a special function of Vipp1. Involvement of diverse ring structures in vesicle formation is suggested. INTRODUCTIONStructural and functional studies of the thylakoid (photosynthetic) membranes of both cyanobacteria and chloroplasts revealed many of their exclusive features. However, the biogenesis and formation of thylakoid membranes is incompletely understood today. Chloroplast thylakoid membranes evolve from vesicles originating from the chloroplast inner envelope membrane, and the formation of thylakoids results in the generation of a completely separated internal membrane system . In contrast, for cyanobacteria it has been a long-standing debate if the internal thylakoid membrane system is physically separated or if it remains connected to the cytoplasmic membrane. Several recent structural studies have, however, indicated that thylakoids are also separated membrane entities in cyanobacteria (Liberton et al., 2006;van de Meene et al., 2006;Nevo et al., 2007;Schneider et al., 2007).For biogenesis of thylakoid membranes many cellular processes, like lipid biosynthesis, membrane formation, protein synthesis in the cytoplasm and/or at a membrane, protein transport, protein translocation, and protein folding have to be organized, chronologically aligned, and controlled. Although for many years genes and proteins specifically involved in formation and biogenesis of thylakoid membranes remained unidentified, in 2001 a protein was described that could be involved in thylakoid membrane biogenesis in plant chloroplasts and cyanobacteria (Kroll et al., 2001;Westphal et al., 2001). The observation that deletion of the open reading frame hcf155 in Arabidopsis thaliana resulted in a complete absence of thylakoid membranes indicated that the encoded protein is involved in thylakoid membrane biogenesis. Further analysis has suggested that the protein might be involved in vesicle trafficking between the inner envelope and thylakoid membranes of chloroplasts (Kroll et al., 2001). Consequently, the protein was named vesicle-inducing protein in plastids 1 (Vipp1). Depletion of Vipp1 in the cyanobacterium Synechocystis PCC 6803 strongly aff...

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