Vol. 11, Issue 1, 131-140, January 2000

Differential Induction of Two p24 Putative Cargo Receptors upon Activation of a Prohormoneproducing Cell

Roland P. Kuiper, Hans R. Waterham,^* Jutta Rötter, Gerrit Bouw, and Gerard J. M. Martens

Department of Animal Physiology, University of Nijmegen, Nijmegen, The Netherlands

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 p24, -, -, and -. We previously identified a member of the p24 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 p24 member (Xp24₂) and on the identification and characterization of a second member (Xp24₁) that is also expressed in the melanotrope cells and that has 66% amino acid sequence identity to Xp24₂. The two p24 members are ubiquitously expressed, but Xp24₂ is neuroendocrine enriched. During black background adaptation, the amount of the Xp24₂ protein in the intermediate pituitary was increased ~25 times, whereas Xp24₁ protein expression was increased only 2.5 times. Furthermore, the level of Xp24₂ mRNA was ~5-fold higher in the melanotrope cells of black-adapted animals than in those of white-adapted animals, whereas Xp24₁ mRNA expression was not induced. Therefore, the expression of Xp24₂ specifically correlates with the expression of POMC. Together, our findings suggest that p24 proteins have a role in selective protein transport in the secretory pathway.