Sorting determinants in the transmembrane domain of p24 proteins

Members of the p24 family of putative cargo receptors are proposed to contain retrograde and anterograde trafficking signals in their cytoplasmic domain to facilitate coat protein binding and cycling in the secretory pathway. We have analyzed the role of the transmembrane domain (TMD) of a p24 protein isolated from COPI-coated intra-Golgi transport vesicles. CD8-p24 chimeras were transiently expressed in COS7 cells and analyzed by immunofluorescence and pulse-chase experiments. The localization and transit of the wild-type chimera from the endoplasmic reticulum (ER) through the Golgi complex involved a glutamic acid residue and a conserved glutamine in the TMD. The TMD glutamic acid mediated the localization of the chimeras to the ER in the absence of the conserved glutamine. Efficient ER exit required the TMD glutamine and was further facilitated by a pair of phenylalanine residues in the cytoplasmic tail. TMD residues of p24 proteins may mediate the interaction with integral membrane proteins of the vesicle budding machinery to ensure p24 packaging into transport vesicles.