Chloroplast vesicle transport

Abstract

Photosynthesis is a well-known process that has been intensively investigated, but less is known about the biogenesis of the thylakoid membrane that harbors the photosynthetic machinery. Thylakoid membranes are constituted by several components, the major ones being proteins and lipids. However, neither of these two are produced in the thylakoid membranes themselves but are targeted there by different mechanisms. The interior of the chloroplast, the stroma, is an aqueous compartment that prevents spontaneous transport of single lipids and/or membrane proteins due to their hydrophobicities. Thylakoid targeted proteins are encoded either in the nucleus or plastid, and thus some cross the envelope membrane before entering one of the identified thylakoid targeting pathways. However, the pathway for all thylakoid proteins is not known. Lipids are produced at the envelope membrane and have been proposed to reach the thylakoid membrane by different means: invaginations of the envelope membrane, direct contact sites between these membranes, or through vesicles. Vesicles have been observed in chloroplasts but not much is yet known about the mechanism or regulation of their formation. The question of whether proteins can also make use of vesicles as one mechanism of transport remains to be answered. Here we discuss the presence of vesicles in chloroplasts and their potential role in transporting lipids and proteins. We additionally discuss what is known about the proteins involved in the vesicle transport and the gaps in knowledge that remain to be filled.

Keywords: Chloroplast; Lipid; Membrane; Targeting; Transport; Vesicles.

Fig. 1

Vesicle appearance in chloroplasts. Chloroplasts from A lettuce (Lactuca sativa) and B Arabidopsis (Arabidopsis thaliana) leaves with vesicles present. E, envelope membrane; T, thylakoid membrane; P, plastoglobules; V, vesicle. Scale bar: 1 µm. Micrograph courtesy to Katalin Solymosi, Eötvös Loránd University, Hungary

Fig. 2

Vesicle transport in chloroplasts. The chloroplast is surrounded by the outer envelope (OE) and inner envelope (IE) membrane, where the latter act as donor membrane for vesicles. The thylakoid membranes consist of grana lamellae and stroma lamellae, and are the target membrane of vesicles. The vesicle process from initiation at the donor membrane to the fusing step at the target membrane is shown. The vesicle is initiated (1) and buds of (2) from the inner envelope where CPSAR1, FZL, THF1, and VIPP1 are found. The vesicle then moves through the stroma (3) where CPRabA5e, CPSAR1 and THF1 are located. Finally, the vesicle gets tethered (4) and fused to the target membrane (5) where CPRabA5e, CURT1, FZL, SCO2, THF1 and VIPP1 are placed