The C-terminus of Bienertia sinuspersici Toc159 contains essential elements for its targeting and anchorage to the chloroplast outer membrane

Most nucleus-encoded chloroplast proteins rely on an N-terminal transit peptide (TP) as a post-translational sorting signal for directing them to the organelle. Although Toc159 is known to be a receptor for specific preprotein TPs at the chloroplast surface, the mechanism for its own targeting and integration into the chloroplast outer membrane is not completely understood. In a previous study, we identified a novel TP-like sorting signal at the C-terminus (CT) of a Toc159 homolog from the single-cell C4 species, Bienertia sinuspersici. In the current study, we have extended our understanding of the sorting signal using transient expression of fluorescently-tagged fusion proteins of variable-length, and with truncated and swapped versions of the CT. As was shown in the earlier study, the 56 residues of the CT contain crucial sorting information for reversible interaction of the receptor with the chloroplast envelope. Extension of this region to 100 residues in the current study stabilized the interaction via membrane integration, as demonstrated by more prominent plastid-associated signals and resistance of the fusion protein to alkaline extraction. Despite a high degree of sequence similarity, the plastid localization signals of the equivalent CT regions of Arabidopsis thaliana Toc159 homologs were not as strong as that of the B. sinuspersici counterparts. Together with computational and circular dichroism analyses of the CT domain structures, our data provide insights into the critical elements of the CT for the efficient targeting and anchorage of Toc159 receptors to the dimorphic chloroplasts in the single-cell C4 species.published_or_final_versio

Isolation of chloroplasts from plant protoplasts

Chloroplasts can be isolated from higher plants directly following homogenization; however, the resulting yield, purity, and intactness are often low, necessitating a large amount of starting materials. This protocol is optimized to produce a high yield of pure chloroplasts from isolated Arabidopsis protoplasts. The two-part method is a simple, scaled-down, and low-cost procedure that readily provides healthy mesophyll protoplasts, which are then ruptured to release intact chloroplasts. Chloroplasts isolated using this method are competent for use in biochemical, cellular, and molecular analyses

Protoplast isolation and staining

The successful isolation of mesophyll protoplasts from plant species has become a versatile tool for in vivo imaging of subcellular structures. Taking advantage of the various cytochemical probes available, the subcellular localization of specific organelles can be visualized in live protoplasts. In an isolated system, monitoring of the dynamics of organelle movement in response to external stimuli, stresses or an exogenous substance can be substantially facilitated. The isolation of a pure population of non-stressed, healthy protoplasts critically affects the reliability and reproducibility of these studies. In this chapter, we detail a standard protocol for the isolation of live mesophyll protoplasts from leaves of the model plant, Arabidopsis thaliana. We also consider the critical factors for empirical optimization of protoplast isolation procedures for succulent species such as Kalanchoe daigremontiana, Bienertia sinuspersici and Lampranthus spectabilis