In Arabidopsis, several isoforms of all the coatomer subunits, except for γ-COP and δ-COP subunits, have been identified. Genes encoding the components of the COPI machinery have been identified in plants ( Robinson et al., 2007 Gao et al., 2014 Ahn et al., 2015 Woo et al., 2015). It has also been reported that the γ subunit interacts with ARF1 and that ζ-COP is required for the stability of γ-COP ( Jackson, 2014). proteins with a dilysine motif) through their N-terminal WD repeat domains. Several studies indicate that the β′-COP and α-COP subunits are involved in binding of cargo (i.e. Following recruitment by the small GTPase ADP-ribosylation factor 1 (ARF1), in its GTP-bound conformation, and uptake of cargo, COPI polymerizes on the membrane surface in such a way that COPI coat assembly depends on both membrane and cargo binding. However, recent structural studies revealed that the subunits are highly connected to each other, suggesting that the COPI structure does not fit with the adaptor F-subcomplex and cage B-subcomplex structure described for other coats ( Dodonova et al., 2015). The B-subcomplex has been proposed to function as the outer layer and the F-subcomplex as the inner layer of the vesicle coat ( Jackson, 2014). It is composed of seven subunits (α/β/β’/γ/δ/ε/ζ) that have been conceptually grouped into two subcomplexes, the B- (α/β The key component of the COPI coat is the coatomer complex, which is essential in eukaryotes and is recruited en bloc onto Golgi membranes ( Hara-Kuge et al., 1994). Many type I transmembrane proteins transported by COPI vesicles bear a C-terminal dilysine-based motif, which has been proven to be recognized by the COPI coat ( Jackson et al. COPI vesicles are formed at the Golgi apparatus and facilitate the retrieval of ER resident proteins from the Golgi to the ER as well as cycling of proteins between ER and the Golgi apparatus. Coat proteins are involved in the selective capture of cargo proteins within the donor compartment, including the fusion machinery needed to ensure vesicle delivery, and the generation of membrane curvature to drive vesicle formation. COPII vesicles are involved in protein export from the ER, whereas COPI vesicles are involved in intra-Golgi transport, where their directionality is still a matter of debate, and in retrograde transport from the Golgi to the ER. The so-called ‘early secretory pathway’ involves bidirectional transport between the ER and the Golgi apparatus, which is mediated by COP (coat protein) I and COPII vesicles ( Brandizzi and Barlowe, 2013). The conventional secretory pathway in plants involves the transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi apparatus and then to the cell surface or vacuole. These findings suggest that loss of α2-COP affects the expression of secretory pathway genes. In addition, a strong upregulation of SEC31A, which encodes a subunit of the COPII coat, was observed in the α2-cop mutant this also occurs in a mutant of a gene upstream of COPI assembly, GNL1, which encodes an ARF-guanine nucleotide exchange factor (GEF). Global expression analysis of the α2-cop mutant revealed altered expression of plant cell wall-associated genes. The α2-cop mutant also exhibited abnormal morphology of the Golgi apparatus. The α2-cop mutant displayed defects in plant growth, including small rosettes, stems and roots and mislocalization of p24δ5, a protein of the p24 family containing a C-terminal dilysine motif involved in COPI binding. To understand the role of COPI proteins in plant biology, we have identified and characterized a loss-of-function mutant of α2-COP, an Arabidopsis α-COP isoform. In contrast to mammals and yeast, several isoforms for coatomer subunits, with the exception of γ and δ, have been identified in Arabidopsis. These vesicles form through the action of the small GTPase ADP-ribosylation factor 1 (ARF1) and the COPI heptameric protein complex (coatomer), which consists of seven subunits (α-, β-, β′-, γ-, δ-, ε- and ζ-COP). COP (coat protein) I-coated vesicles mediate intra-Golgi transport and retrograde transport from the Golgi to the endoplasmic reticulum.
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