An Arabidopsis GRIP domain protein locates to the trans-Golgi and binds the small GTPase ARL1


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GRIP domain proteins are a class of golgins that have been described in yeast and animals. They locate to the trans-Golgi network and are thought to play a role in endosome-to-Golgi trafficking. The Arabidopsis GRIP domain protein, AtGRIP, fused to the green fluorescent protein (GFP), locates to Golgi stacks but does not exactly co-locate with the Golgi marker sialyl transferase (ST)-mRFP, nor with the t-SNAREs Memb11, SYP31 and BS14a. We conclude that the location of AtGRIP is further to the trans side of the stack than STtmd–mRFP. The 185-aa C-terminus of AtGRIP containing the GRIP domain targeted GFP to the Golgi, although a proportion of the fusion protein was still found in the cytosol. Mutation of a conserved tyrosine (Y717) to alanine in the GRIP domain disrupted Golgi localization. ARL1 is a small GTPase required for Golgi targeting of GRIP domain proteins in other systems. An Arabidopsis ARL1 homologue was isolated and shown to target to Golgi stacks. The GDP-restricted mutant of ARL1, AtARL1-T31N, was observed to locate partially to the cytosol, whereas the GTP-restricted mutant AtARL1-Q71L labelled the Golgi and a population of small structures. Increasing the levels of AtARL1 in epidermal cells increased the proportion of GRIP–GFP fusion protein on Golgi stacks. We show, moreover, that AtARL1 interacted with the GRIP domain in a GTP-dependent manner in vitro in affinity chromatography and in the yeast two-hybrid system. This indicates that AtGRIP and AtARL1 interact directly. We conclude that the pathway involving ARL1 and GRIP domain golgins is conserved in plants.