NUCLEOLAR AND NUCLEAR-LOCALIZATION PROPERTIES OF A HERPESVIRUS BZIP ONCOPROTEIN, MEQ

Marek's disease virus (MDV) is one of the most oncogenic herpesviruses and induces T lymphomas in chickens within weeks after infection. Onl y a limited number of viral transcripts are detected in MDV tumor samp les and cell lines. One of the major transcripts encodes MEQ, a 339-am ino-acid bZIP protein which is homologous to the Jun/Fos family of tra nscription factors. The C-terminal half of MEQ contains proline-rich r epeats and, when fused to the DNA-binding domain of a yeast transcript ion factor, Gal4 (residues 1 to 147), exhibits transactivation functio n. MEQ can dimerize with itself and with c-Jun. The MEQ-c-Jun heterodi mers bind to an AP-1-like enhancer within the MEQ promoter region with greater affinity than do homodimers of either protein, and they trans activate MEQ expression. Here we show that MEQ is expressed in the nuc leus but, interestingly, with a predominant fraction in the nucleoli a nd coiled bodies. This makes MEQ the first bZIP protein to be identifi ed in the nucleoli. MEQ contains two stretches of basic residues, desi gnated basic region 1 (BR1) and basic region 2 (BR2). Using a series o f deletion mutants, we have mapped the primary nuclear localization si gnal (NLS) and the sole nucleolar localization signal (NoLS) to the BR 2 region. BR1 was shown to provide an auxiliary signal in nuclear tran slocation. To demonstrate that BR2 is an authentic NoLS, BR2 was fused to cytoplasmic v-Raf (Delta gag) kinase. The BR2-Raf fusion protein w as observed to migrate into the nucleoplasm and the nucleolus. The BR2 region can be further divided into two long arginine-lysine stretches , BR2N and BR2C, which are separated by the five amino acids Asn-Arg-A sp-Ala-Ala (NRDAA). We provide evidence that the requirement for nucle ar translocation is less stringent than that for nucleolar translocati on, as either BR2N or BR2C alone is sufficient to translocate the cyto plasmic v-Raf (Delta gag) into the nucleus, but only in combination ca n they translocate v-Raf (Delta gag) into the nucleolus. Our studies d emonstrate that MEQ is both a nuclear and nucleolar protein, adding ME Q to the growing list of transactivators which localize to the nucleol us.