TRANSLOCON-ASSOCIATED PROTEIN TRAP-DELTA AND A NOVEL TRAP-LIKE PROTEIN ARE COORDINATELY EXPRESSED WITH PROOPIOMELANOCORTIN IN XENOPUS INTERMEDIATE PITUITARY

In the intermediate pituitary gland of Xenopus laevis, the expression levels of the prohormone pro-opiomelanocortin (POMC) can be readily ma nipulated. When the animal is placed on a black background, the gene f or POMC is actively transcribed, whereas on a white background the gen e is virtually inactive. In this study, we characterized two genes who se transcript levels in the intermediate pituitary are regulated in co ordination with that for POMC. One of these codes for a protein homolo gous to translocon-associated protein TRAP delta, a subunit of a trans membrane protein complex located at the site where nascent secretory p roteins enter the endoplasmic reticulum (ER). Both Xenopus and mice we re found to express an alternatively spliced transcript that gives ris e to a previously unknown version of the TRAPS protein. The product of the second gene is a novel and highly conserved protein with structur al similarity to glycoprotein gp25L, a constituent of another transloc on-associated protein complex. A database search revealed the existenc e of a novel family of gp25L-related proteins whose members occur thro ughout the animal kingdom. Together, our data imply that (i) the group of ER proteins surrounding translocating polypeptide chains may be fa r more complex than previously expected, and (ii) a number of the acce ssory components of the translocon participate in early steps of proho rmone biosynthesis.