In vivo trafficking and localization of p24 proteins in plant cells

p24 proteins constitute a family of putative cargo receptors that traffic in the early secretory pathway. p24 proteins can be divided into four subfamilies (p23, p24, p25 and p26) by sequence homology. In contrast to mammals and yeast, most plant p24 proteins contain in their cytosolic C-terminus both a dilysine motif in the -3, -4 position and a diaromatic motif in the -7, -8 position. We have previously shown that the cytosolic tail of Arabidopsis p24 proteins has the ability to interact with ARF1 and coatomer (through the dilysine motif) and with COPII subunits (through the diaromatic motif). Here, we establish the localization and trafficking properties of an Arabidopsis thaliana p24 protein (Atp24) and have investigated the contribution of the sorting motifs in its cytosolic tail to its in vivo localization. Atp24-red fluorescent protein localizes exclusively to the endoplasmic reticulum (ER), in contrast with the localization of p24 proteins in other eukaryotes, and the dilysine motif is necessary and sufficient for ER localization. In contrast, Atp24 mutants lacking the dilysine motif are transported along the secretory pathway to the prevacuolar compartment and the vacuole, although a significant fraction is also found at the plasma membrane. Finally, we have found that ER export of Atp24 is COPII dependent, while its ER localization requires COPI function, presumably for efficient Golgi to ER recycling.