Fibril Assembly and Carotenoid Overaccumulation in Chromoplasts: A Model for Supramolecular Lipoprotein Structures

Abstract

Chromoplast development in ripening bell pepper fruits is characterized by a massive synthesis of carotenoid pigments, resulting in their distinctive red color. We have shown that 95% of these pigments accumulate in chromoplasts in specific lipoprotein fibrils. In addition to carotenoids, purified fibrils contain galactolipids, phospholipids, and a single, 32-kD protein, designated fibrillin, which has antigenically related counterparts in other species. Fibrils were reconstituted in vitro when purified fibrillin was combined with carotenoids and polar lipids in the same stoichiometric ratio found in fibrils in vivo. Antibodies directed against fibrillin were used to isolate a fibrillin cDNA clone and, in immunological studies, to follow its accumulation during the chloroplast-to-chromoplast transition under different conditions. A model for fibril architecture is proposed wherein carotenoids accumulate in the center of the fibrils and are surrounded by a layer of polar lipids, which in turn are surrounded by an outer layer of fibrillin. Topological analysis of purified fibrils verified this structure. Collectively, these results suggest that the process of fibril self-assembly in chromoplasts is an example of a general phenomenon shared among cells that target excess membrane lipids into deposit structures to avoid their destabilizing or toxic effects. In addition, we have shown that abscisic acid stimulates this phenomenon in chromoplasts, whereas gibberellic acid and auxin delay it.

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

“…In chromoplast PGs, we identified homologs of FIB1a, 1b, 2, and 4. Fibrillins have one or more hydrophobic regions, which have been proposed to play a role in stabilizing the globules and preventing coalescence (Deruere et al, 1994). Importantly, Camara, Kuntz, and colleagues (Deruere et al, Identification of the protein by peptide mass fingerprinting from 1-D PAGE is indicated with the number of the gel band (from the gel in Fig.…”

“…PGs were also proposed to play a role in the removal of protein catabolites as part of thylakoid turnover (Ghosh et al, 1994;Smith et al, 2000). In chromoplasts, PGs are the main storage and remodeling site for carotenoids (Deruere et al, 1994). So far, it is not known whether PG proteomes include any enzymes.…”

“…PGs in chromoplasts of red bell pepper (Capsicum annuum) fruits, as well as colored flower petals, accumulate high levels of carotenoids (xanthophyll esters), a-tocopherol, and some plastoquinone and often have a fibrillar or tubular shape (Emter et al, 1990;Deruere et al, 1994). Their carotenoids, tocopherols, and quinones are sequestered in the central core, and the hydrophobic tails of the polar galacto-and phospholipids are oriented in a monolayer toward the core, whereas their polar head groups are oriented outward, interacting with a protein coat (Deruere et al, 1994).…”

Plastoglobule Proteome

“…There are 11 or more fibrillin proteins in land plants associated with both plastoglobules and thylakoid membranes. They have one or more hydrophobic regions and are proposed to stud the surface of the plastoglobule to prevent them from coalescing (Deruere et al, 1994). Fibrillin expression increased following the high-light exposure, potentially corresponding to the observed increase in plastoglobule number during stress conditions.…”

“…Etioplasts have large numbers of plastoglobules, and increase in plastoglobule size and numbers is also observed during of senescence (for review, see Hopkins et al, 2007). In chloroplasts, plastoglobules contain quinones, a-tocopherol, and lipids, and those in chromoplasts accumulate high levels of carotenoids (Deruere et al, 1994). Other than the structural proteins fibrillins (described below), little is known about the protein composition of plastoglobules.…”