Subclass-Specific Localization and Trafficking of Arabidopsis p24 Proteins in the ER-Golgi Interface

We describe a comprehensive analysis of the subcellular localization and in vivo trafficking of Arabidopsis p24 proteins. In Arabidopsis, there are 11 p24 proteins, which fall into only delta and beta subfamilies. Interestingly, the d subfamily of p24 proteins in Arabidopsis is elaborated spectacularly in evolution, which can be grouped into two subclasses: p24 delta 1 and p24 delta 2. We found that, although all p24 delta proteins possess classic COPII/COPI binding motifs in their cytosolic C-termini, p24 delta 1 proteins are localized to the endoplasmic reticulum (ER), p24 delta 2 proteins are localized to both ER and Golgi. Two p24 beta proteins reside largely in Golgi. Similar to Atp24 (termed p24 delta 1c in this study), p24 delta 2d also cycles between the ER and Golgi. Interestingly, coexpression with p24 beta 1 could retain p24 delta 2d, but not p24 delta 1d in Golgi. We revealed that the lumenal coiled-coil domain of p24 delta 2d is required for its steady-state localization in Golgi, probably through its interaction with p24 beta 1. In p24 beta 1, there is no classic COPII or COPI binding motif in its C-terminus. However, the protein also cycles between the ER and Golgi. We found that a conserved RV motif located at the extreme end of the C-terminus of p24 beta 1 plays an important role in its Golgi target.