Coupled transport of Arabidopsis p24 proteins at the ER-Golgi interface

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24 alpha, p24 beta, p24 gamma, and p24 delta. In contrast to animals and fungi, plants contain only members of the p24 beta and p24 delta subfamilies. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24 delta 5 localizes to the endoplasmic reticulum (ER) as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. Using specific antibodies, endogenous p24 delta 5 has now been localized to the ER and p24 beta 2 to the Golgi apparatus in Arabidopsis root tip cells by immunogold electron microscopy. The relative contributions of the cytosolic tail and the luminal domains to p24 delta 5 trafficking have also been characterized. It is demonstrated that whereas the dilysine motif in the cytoplasmic tail determines the location of p24 delta 5 in the early secretory pathway, the luminal domain may contribute to its distribution downstream of the Golgi apparatus. By using knock-out mutants and co-immunoprecipitation experiments, it is shown that p24 delta 5 and p24 beta 2 interact with each other. Finally, it is shown that p24 delta 5 and p24 beta 2 exhibit coupled trafficking at the ER-Golgi interface. It is proposed that p24 delta 5 and p24 beta 2 interact with each other at ER export sites for ER exit and coupled transport to the Golgi apparatus. Once in the Golgi, p24 delta 5 interacts very efficiently with the COPI machinery for retrograde transport back to the ER.