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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, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24[delta]5 (p24[delta]1 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 RFP-p24[delta]9 (p24[delta]2 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)-p24[beta]3 mainly localizes to the Golgi apparatus (as p24[beta]2) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p24[delta]9 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p24[beta]3 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 [beta] and [delta] subfamilies) which are important for their stability and their coupled trafficking at the ER-Golgi interface. Evidence is also provided for a role for p24[delta]5 in retrograde Golgi-ER transport of the KDEL-receptor ERD2.

(C) Society for Experimental Biology 2013. Published by Oxford University Press. All rights reserved.