Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway

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, p24, p24, and p24. In contrast to animals and fungi, plants contain only members of the p24 and p24 subfamilies, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)p245 (p241 subclass) localizes to the endoplasmic reticulum (ER) at steady state as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. It is now shown that transiently expressed RFPp249 (p242 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)p243 mainly localizes to the Golgi apparatus (as p242) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p249 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p243 mainly localizes to the Golgi apparatus. By a combination of experiments using transient expression, knock-out mutants, and co-immunoprecipitation, it is proposed that Arabidopsis p24 proteins form different heteromeric complexes (including members of the and subfamilies) which are important for their stability and their coupled trafficking at the ERGolgi interface. Evidence is also provided for a role for p245 in retrograde GolgiER transport of the KDEL-receptor ERD2.