Differential Induction of Two p24δ Putative Cargo Receptors upon Activation of a Prohormoneproducing Cell

The p24 family consists of type I transmembrane proteins that are present abundantly in transport vesicles, may play a role in endoplasmic reticulum-to-Golgi cargo transport, and have been classified into subfamilies named p24α, -β, -γ, and -δ. We previously identified a member of the p24δ subfamily that is coordinately expressed with the prohormone proopiomelanocortin (POMC) in the melanotrope cells of the intermediate pituitary during black background adaptation of the amphibian Xenopus laevis (∼30-fold increase in POMC mRNA). In this study, we report on the characterization of this p24δ member (Xp24δ(2)) and on the identification and characterization of a second member (Xp24δ(1)) that is also expressed in the melanotrope cells and that has 66% amino acid sequence identity to Xp24δ(2). The two p24δ members are ubiquitously expressed, but Xp24δ(2) is neuroendocrine enriched. During black background adaptation, the amount of the Xp24δ(2) protein in the intermediate pituitary was increased ∼25 times, whereas Xp24δ(1) protein expression was increased only 2.5 times. Furthermore, the level of Xp24δ(2) mRNA was ∼5-fold higher in the melanotrope cells of black-adapted animals than in those of white-adapted animals, whereas Xp24δ(1) mRNA expression was not induced. Therefore, the expression of Xp24δ(2) specifically correlates with the expression of POMC. Together, our findings suggest that p24δ proteins have a role in selective protein transport in the secretory pathway.