Abstract
Coat protein I (COPI) is necessary for intra-Golgi transport and retrograde transport from the Golgi apparatus back to the endoplasmic reticulum. The key component of the COPI coat is the coatomer complex, which is composed of seven subunits (α/β/β’/γ/δ/ε/ζ) and is recruited en bloc from the cytosol onto Golgi membranes. In mammals and yeast, α- and β’-COP WD40 domains mediate cargo-selective interactions with dilysine motifs present in canonical cargoes of COPI vesicles. In contrast to mammals and yeast, three isoforms of β’-COP (β’1-3-COP) have been identified in Arabidopsis. To understand the role of Arabidopsis β’-COP isoforms in plant biology, we have identified and characterized loss-of-function mutants of the three isoforms, and double mutants were also generated. We have found that the trafficking of a canonical dilysine cargo (the p24 family protein p24δ5) is affected in β’-COP double mutants. By western blot analysis, it is also shown that protein levels of α-COP are reduced in the β’-COP double mutants. Although none of the single mutants showed an obvious growth defect, double mutants showed different growth phenotypes. The double mutant analysis suggests that, under standard growth conditions, β’1-COP can compensate for the loss of both β’2-COP and β’3-COP and may have a prominent role during seedling development.
Highlights
Coat Protein I (COPI)-coated vesicles are involved in transport between Golgi cisternae and in retrograde transport from the Golgi apparatus back to the endoplasmic reticulum (ER) [1]
The COPI coat is based on a cytosolic complex, containing seven equimolar subunits (α, β, β’, γ, δ, ε– and ζ-COP), which interacts with Golgi membranes via the GTPase ADP-ribosylation factor 1 (ARF1)
The main difference between the three isoforms is their expression during seed development
Summary
Coat Protein I (COPI)-coated vesicles are involved in transport between Golgi cisternae and in retrograde transport from the Golgi apparatus back to the endoplasmic reticulum (ER) [1]. COPI proteins have been recently proposed as playing a role in the last step of anterograde ER–Golgi transport [2,3]. The COPI coat is based on a cytosolic complex (coatomer), containing seven equimolar subunits (α–, β–, β’–, γ–, δ–, ε– and ζ-COP), which interacts with Golgi membranes via the GTPase ADP-ribosylation factor 1 (ARF1). Cytosolic (GDP-bound) ARF1 first interacts with dimers of p24 family proteins, but following GTP/GDP exchange, ARF1–GTP dissociates from p24 proteins and inserts into Golgi membranes. Coatomer can interact both with ARF1–GTP and with sorting signals (i.e., dilysine motifs) in the cytosolic domain of p24 family proteins and other COPI cargo proteins.
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