The puzzle of chloroplast vesicle transport – involvement of GTPases

Abstract

In the cytosol of plant cells vesicle transport occurs via secretory pathways among the endoplasmic reticulum network, Golgi bodies, secretory granules, endosome, and plasma membrane. Three systems transfer lipids, proteins and other important molecules through aqueous spaces to membrane-enclosed compartments, via vesicles that bud from donor membranes, being coated and uncoated before tethered and fused with acceptor membranes. In addition, molecular, biochemical and ultrastructural evidence indicates presence of a vesicle transport system in chloroplasts. Little is known about the protein components of this system. However, as chloroplasts harbor the photosynthetic apparatus that ultimately supports most organisms on the planet, close attention to their pathways is warranted. This may also reveal novel diversification and/or distinct solutions to the problems posed by the targeted intra-cellular trafficking of important molecules. To date two homologs to well-known yeast cytosolic vesicle transport proteins, CPSAR1 and CPRabA5e (CP, chloroplast localized), have been shown to have roles in chloroplast vesicle transport, both being GTPases. Bioinformatic data indicate that several homologs of cytosolic vesicle transport system components are putatively chloroplast-localized and in addition other proteins have been implicated to participate in chloroplast vesicle transport, including vesicle-inducing protein in plastids 1, thylakoid formation 1, snowy cotyledon 2/cotyledon chloroplast biogenesis factor, curvature thylakoid 1 proteins, and a dynamin like GTPase FZO-like protein. Several putative potential cargo proteins have also been identified, including building blocks of the photosynthetic apparatus. Here we discuss details of the largely unknown putative chloroplast vesicle transport system, focusing on GTPase-related components.

Envelope K+/H+ Antiporters AtKEA1 and AtKEA2 Function in Plastid Development

“…In accordance, AtKEA1 and AtKEA2 proteins are present in large protein complexes of unknown functions at the inner envelope (Roston et al, 2012). Coiled-coil proteins play important roles in chloroplast division, thylakoid membrane development, and chloroplast relocation in response to light, by interaction with cytoskeleton-like elements (Wada, 2013;Karim and Aronsson, 2014;Osteryoung and Pyke, 2014). Furthermore, although the AtKEA2 protein without the N-terminal domain constitutes an active antiporter (Aranda-Sicilia et al, 2012), it fails to complement the growth defects of the double mutant plant, suggesting that the specific localization of the protein is crucial for its function.…”

Proteins affecting thylakoid morphology – the key to understanding vesicle transport in chloroplasts?

“…However, this explanation currently remains a hypothesis, but reports of proteins altering the thylakoid membrane formation and organization are indeed accumulating. 28 To summarize, mutants of CPRabA5e, CURT1A and C, THF1 and SCO2 all show accumulation of vesicles, indicating their direct/indirect involvement in fusion of vesicles to thylakoid membranes. The VIPP1 mutant, on the other hand, is unable to form vesicles, indicating direct/ indirect involvement in budding events.…”

Protein biocargo of citrus fruit-derived vesicles reveals heterogeneous transport and extracellular vesicles populations

“…Physiochemical characterization of different vesicle populations: 3 4 Samples were diluted to 1 µg/µL concentration using 0.1 M PBS pH 7.6 and five microliter aliquots 5 were used for TEM analysis. The samples were deposited onto formvar and carbon coated 300 mesh 6 copper grids for one minute.…”