Differential induction of two p24 delta putative cargo receptors upon activation of a prohormone-producing cell

The p24 family consists of type I transmembrane proteins that are present a bundantly in transport vesicles, may play a role in endoplasmic reticulum-t o-Golgi cargo transport, and have been classified into subfamilies named p2 4 alpha, -beta, -gamma, and -delta. We previously identified a member of th e p246 subfamily that is coordinately expressed with the prohormone proopio melanocortin (POMC) in the melanotrope cells of the intermediate pituitary during black background adaptation of the amphibian Xenopus laevis (similar to 30-fold increase in POMC mRNA). In this study, we report on the charact erization of this p24 delta member (Xp24 delta(2)) and on the identificatio n and characterization of a second member (Xp24 delta(1)) that is also expr essed in the melanotrope cells and that has 66% amino acid sequence identit y to Xp24 delta(2). The two p24 delta members are ubiquitously expressed, b ut Xp24 delta(2) is neuroendocrine enriched. During black background adapta tion, the amount of the Xp24 delta(2) protein in the intermediate pituitary was increased similar to 25 times, whereas Xp24 delta(1) protein expressio n was increased only 2.5 times. Furthermore, the level of Xp24 delta(2) mRN A was similar to 5-fold higher in the melanotrope cells of black-adapted an imals than in those of white-adapted animals, whereas Xp24 delta(1) mRNA ex pression was not induced. Therefore, the expression of Xp24 delta(2) specif ically correlates with the expression of POMC. Together, our findings sugge st that p24 delta proteins have a role in selective protein transport in th e secretory pathway.