The p24␣, -␤, -␥, and -␦ proteins are major multimeric constituents of cycling endoplasmic reticulum-Golgi transport vesicles and are thought to be involved in protein transport through the early secretory pathway. In this study, we targeted transgene overexpression of p24␦ 2 specifically to the Xenopus intermediate pituitary melanotrope cell that is involved in background adaptation of the animal and produces high levels of its major secretory cargo proopiomelanocortin (POMC). The transgene product effectively displaced the endogenous p24 proteins, resulting in a melanotrope cell p24 system that consisted predominantly of the transgene p24␦ 2 protein. Despite the severely distorted p24 machinery, the subcellular structures as well as the level of POMC synthesis were normal in these cells. However, the number and pigment content of skin melanophores were reduced, impairing the ability of the transgenic animal to fully adapt to a black background. This physiological effect was likely caused by the affected profile of POMC-derived peptides observed in the transgenic melanotrope cells. Together, our results suggest that in the early secretory pathway an intact p24 system is essential for efficient secretory cargo transport or for supplying cargo carriers with the correct protein machinery to allow proper secretory protein processing. INTRODUCTIONTransport of cargo proteins through the early secretory pathway involves cargo selection, transport vesicle formation, quality control to recycle misfolded cargo, and cycling of the COPI-and COPII-coated vesicles between the endoplasmic reticulum (ER) and Golgi (Barlowe, 2000). One of the major constituents of the transport vesicles is the p24 family of type I transmembrane proteins that can be classified into four main subfamilies, designated p24␣, -␤, -␥, and -␦ (Schimmö ller et al., 1995;Stamnes et al., 1995;Sohn et al., 1996;Nickel et al., 1997;Dominguez et al., 1998). The p24 proteins share a number of structural characteristics, such as a relatively large lumenal putative cargo-binding domain, a coiled-coil region thought to be involved in the formation of multimeric p24 complexes, a transmembrane region, and a short cytoplasmic tail containing COPI-and COPII-binding motifs that are used for p24 traveling from the ER to the Golgi and back (for review, see Kaiser, 2000). In yeast and mammalian cells, p24 proteins form functional heterotetrameric complexes containing one representative of each subfamily, whereby the composition of the complex may differ in various cell types (Dominguez et al., 1998;Fü llekrug et al., 1999;Marzioch et al., 1999;Ciufo and Boyd, 2000;Emery et al., 2000;Belden and Barlowe, 2001). Furthermore, the stability of the p24 members seems to be compromised when cells are deficient in the expression of a single p24 protein (Marzioch et al., 1999;Denzel et al., 2000). Recent evidence suggests a complex and dynamic p24 system of mostly monomers and homo-/heterodimers and that the degree of oligomerization constantly alters and largely depends on the s...