Variant Max protein, derived by alternative splicing, associates with c-Myc in vivo and inhibits transactivation.
Max (Myc-associated factor X) is a basic helix-loop-helix/leucine zipper protein that has been shown to play a central role in the functional activity of c-Myc as a transcriptional activator. Max potentiates the binding of Myc-Max heterodimers through its basic region to its specific E-box Myc site (EMS), enabling c-Myc to transactivate effectively. In addition to the alternatively spliced exon a, several naturally occurring forms of alternatively spliced max mRNAs have been reported, but variant protein products from these transcripts have not been detected. Using Western blot (immunoblot) and immunoprecipitation analysis, we have identified a variant form of Max protein (16 to 17 kDa), termed dMax, in detergent nuclear extracts of murine B-lymphoma cells, normal B lymphocytes, and NIH 3T3 fibroblasts. Cloning and sequencing revealed that dMax contains a deletion spanning the basic region and helix 1 and the loop of the helix-loop-helix region, presumably as a result of alternative splicing of max RNA. S1 nuclease analysis confirmed the presence of the mRNA for dMax in cells. The dMax protein, prepared via in vitro transcription and translation, associated with bacterially synthesized Myc-glutathione S-transferase. Coimmunoprecipitation of dMax and c-Myc indicated their intracellular association. In vitro-synthesized dMax failed to bind EMS DNA, presumably because of the absence of the basic region. Coexpression of dMax inhibited EMS-mediated transactivation by c-Myc. Thus dMax, which can interact with c-Myc, appears to function as a dominant negative regulator, providing an additional level of regulation to the transactivation potential of c-Myc.
Arsura
M
M
Department of Biochemistry, Boston University School of Medicine, Massachusetts 02118-2394, USA.
Deshpande
A
A
Hann
S R
SR
Sonenshein
G E
GE
D016475:3T3 Cells; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D050976:Basic-Leucine Zipper Transcription Factors; D015153:Blotting, Western; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010957:Plasmids; D016133:Polymerase Chain Reaction; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D015533:Transcriptional Activation
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D050976:Basic-Leucine Zipper Transcription Factors; D017931:DNA Primers; D004268:DNA-Binding Proteins; C499330:MAX protein, human; C067850:Myc associated factor X; D016271:Proto-Oncogene Proteins c-myc; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; C068422:Max protein, mouse