2 PROTEINS TRANSLATED BY ALTERNATIVE USAGE OF INITIATION CODONS IN MESSENGER-RNA ENCODING A JUND TRANSCRIPTIONAL REGULATOR

The junD gene encodes one component of the transcription factor, AP-1. Since two forms of JunD protein have been reported, we analyzed here the molecular mechanisms involved in the isoform production. Immunoche mical analysis indicated that the longer and shorter forms of mouse Ju nD (JunD-L and JunD-S, with apparent molecular weights of 44 and 39 kD a, respectively) differ in their content of an N-terminal peptide. Mut ational analysis further indicated that JunD-S is the translational pr oduct initiated at the third AUG located 144 bp from the first AUG, at which JunD-L translation starts. Such production of two junD isoforms from a single mRNA using the same reading frame seems to be conserved in human, rat, and chicken. To examine the functional differences bet ween the isoforms, each type of JunD was exclusively expressed by the use of retrovirus vectors harboring the mutated junD gene, The exogeno us expression of either one of these forms did not cause cellular tran sformation of NIH3T3, but suppressed the anchorage-independent growth of NIH3T3 transforbold by the activated K-ras or v-src gene. These two isoforms were expressed in all the mouse tissues examined and in vari ous cell lines established from human tumors, though the expression ra tio between JunD-L and JunD-S varied, suggesting that some factor(s) m odulate the alternative usage of the initiation codon of the junD gene . (C) 1998 Academic Press.