ACTIVATION OF GENE-TRANSCRIPTION BY THE AMINO-TERMINUS OF THE N-MYC PROTEIN DOES NOT REQUIRE ASSOCIATION WITH THE PROTEIN ENCODED BY THE RETINOBLASTOMA SUPPRESSOR GENE RB1
C. Cziepluch et al., ACTIVATION OF GENE-TRANSCRIPTION BY THE AMINO-TERMINUS OF THE N-MYC PROTEIN DOES NOT REQUIRE ASSOCIATION WITH THE PROTEIN ENCODED BY THE RETINOBLASTOMA SUPPRESSOR GENE RB1, Oncogene, 8(10), 1993, pp. 2833-2838
N-Myc encodes a nuclear phosphoprotein that contains a basic region (B
R), a helix-loop-helix (HLH) and a leucine zipper (Zip). These motifs
are hallmarks of certain transcription factors. In pursuit of the ques
tion if N-Myc can activate transcription, we have employed an experime
ntal model involving the yeast transcription factor Gal4. We have firs
t generated fusion proteins containing the Gal4 DNA-binding domain joi
ned to portions of N-Myc. Subsequently we have analysed if chimeric pr
oteins can transactivate the transcription of a reporter under the con
trol of Gal4 binding sites. Here we show that the amino terminal porti
on of N-Myc activates transcription. Activation maps to a domain highl
y conserved among Myc-proteins and to other non-conserved sequences, s
uggesting functional redundancy. Previous studies had documented in vi
tro association of the RBI protein with N-Myc (Rustig et al., 1991). W
e here confirm this observation and identify the region encompassing t
he transactivation domain as responsible for RBI binding. Analyses of
N-Myc transactivation in retinoblastoma cell line WERI lacking a funct
ion RBI protein gave results similar to those with cell lines having a
n intact RBI protein, showing that RBI protein is not required for tra
nsactivation by N-Myc. The present findings leave open the question if
deregulated expression of N-Myc contributes to tumorigenesis by trans
criptional activation of as yet unidentified target genes or by functi
onally inactivating the protein encoded by the tumor suppressor gene R
B1, or by a combination of both.