COP-binding sites in p24δ2 are necessary for proper secretory cargo biosynthesis

The p24 family is thought to be somehow involved in endoplasmic reticulum-to-Golgi protein transport, and its members are major constituents of transport vesicles and bind to the vesicle coat protein complexes COPI and COPII. A subset of the p24 proteins (p24 alpha(3), -beta(1), -gamma(3) and -delta(2)) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC). To investigate the role of the COP-binding motifs of p24 proteins in POMC biosynthesis, we here generated and analysed Xenopus with stable, melanotrope cell-specific transgene expression of p24 delta(2)-GFP mutated in its COPI- or COPII-binding motif In contrast to what has been found previously for wild-type (wt) p24 delta(2)-GFP, the p24 delta(2) mutations prevented the Golgi localisation of the transgene products and caused a reduced rate of POMC cleavage, but did not lead to a reduction of the endogenous p24 proteins nor to aberrations in POW glycosylation and sulphation. We conclude that p24 delta(2) requires the presence of the COPI- and COPII-binding sites to allow proper POMC processing. Thus, the p24 proteins fulfil their role in secretory protein biosynthesis via COPI- or COPII-coated transport vesicles. (c) 2009 Elsevier Ltd. All rights reserved.