Protein Profiling of Plastoglobules in Chloroplasts and Chromoplasts. A Surprising Site for Differential Accumulation of Metabolic Enzymes

Abstract

Plastoglobules (PGs) are oval or tubular lipid-rich structures present in all plastid types, but their specific functions are unclear. PGs contain quinones, a-tocopherol, and lipids and, in chromoplasts, carotenoids as well. It is not known whether PGs contain any enzymes or regulatory proteins. Here, we determined the proteome of PGs from chloroplasts of stressed and unstressed leaves of Arabidopsis (Arabidopsis thaliana) as well as from pepper (Capsicum annuum) fruit chromoplasts using mass spectrometry. Together, this showed that the proteome of chloroplast PGs consists of seven fibrillins, providing a protein coat and preventing coalescence of the PGs, and an additional 25 proteins likely involved in metabolism of isoprenoid-derived molecules (quinines and tocochromanols), lipids, and carotenoid cleavage. Four unknown ABC1 kinases were identified, possibly involved in regulation of quinone monooxygenases. Most proteins have not been observed earlier but have predicted N-terminal chloroplast transit peptides and lack transmembrane domains, consistent with localization in the PG lipid monolayer particles. Quantitative differences in PG composition in response to high light stress and degreening were determined by differential stable-isotope labeling using formaldehyde. More than 20 proteins were identified in the PG proteome of pepper chromoplasts, including four enzymes of carotenoid biosynthesis and several homologs of proteins observed in the chloroplast PGs. Our data strongly suggest that PGs in chloroplasts form a functional metabolic link between the inner envelope and thylakoid membranes and play a role in breakdown of carotenoids and oxidative stress defense, whereas PGs in chromoplasts are also an active site for carotenoid conversions.

A mechanism implicating plastoglobules in thylakoid disassembly during senescence and nitrogen starvation

“…Among the proteins of the Arabidopsis PG proteome, ABC1-like kinases are among the most abundant Ytterberg et al 2006;Lundquist et al 2012b). Even though little is known about functions of the plastoglobule ABC1-like kinases in plants, the prototypical function of this family is the regulation of ubiquinone metabolism in bacteria and mitochondria (Lundquist et al 2012a).…”

“…Both were linked to oxidative stress under high light conditions. Thus, the chloroplast localization together with the phenotypes and the analogy to mitochondrial and bacterial ABC1-like kinases suggests that plastoglobule ABC1-like kinase proteins regulate prenylquinone metabolism and that they may do so via phosphorylation of enzymes in the pathway (Ytterberg et al 2006;Lundquist et al 2012a). Indeed, in yeast, it has been demonstrated that ABC1/Coq8 is required for the phosphorylation of Coq3, Coq5 and Coq7, three enzymes involved in ubiquinone pathway.…”

“…Further indications for the implication of plastoglobules in carotenoid metabolism stems come from the identification of z-carotene desaturase (ZDS), lycopene b-cyclase (LYC-b or CYC-b), and two b-carotene b-hydroxylases (CrtR-b) in the PG chromoplast proteome of tomato (Ytterberg et al 2006) (Fig. 1c).…”

“…However, since the characterization of the plastoglobule proteome from Arabidopsis chloroplasts and red-pepper chromoplasts, this idea has changed. The proteomics studies revealed the presence of proteins that can be attributed to three different groups: (1) structural proteins called fibrillins (FBN) or plastoglobulins (PGL), (2) enzymes involved in various lipid metabolic pathways and (3) uncharacterized proteins Ytterberg et al 2006;Lundquist et al 2012b).…”

Dual targeting of a mature plastoglobulin/fibrillin fusion protein to chloroplast plastoglobules and thylakoids in transplastomic tobacco plants

“…The proteome of plastoglobules has around two dozen proteins Ytterberg et al 2006;Lundquist et al 2012). These belong to three categories: plastoglobulins/fibrillins (PGL or FBN), plastid metabolic proteins and unclassified proteins.…”