The alternatively initiated c-Myc proteins differentially regulate transcription through a noncanonical DNA-binding site

Alternative translation initiation site usage results in two functionally distinct forms of the GATA-1 transcription factor

GATA-1 is a zinc-finger transcription factor that plays a critical role in the normal development of hematopoietic cell lineages. In human and murine erythroid cells a previously undescribed 40-kDa protein is detected with GATA-1-specific antibodies. We show that the 40-kDa GATA-1 (GATA-1s) is produced by the use of an internal AUG initiation codon in the GATA-1 transcript. The GATA-1 proteins share identical binding activity and form heterodimers in erythroleukemic cells but differ in their transactivation potential and in their expression in developing mouse embryos.

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.

Hoxa 11 structure, extensive antisense transcription, and function in male and female fertility

Hoxa 11 is a murine Abdominal-B-type homeobox gene. The structure of this gene is presented, including genomic and cDNA sequence. The cDNA includes the complete open reading frame and based on primer extension results is near full length. Surprisingly, the antisense strand of Hoxa 11 was found to be transcribed. Moreover, these antisense transcripts were processed and polyadenylated. The developmental expression patterns for both sense and antisense transcripts were examined using serial section and whole-mount in situ hybridizations. Hoxa 11 transcription patterns were defined in the limbs, kidney and stromal cells surrounding the Mullerian and Wolffian ducts. Of particular interest, in the developing limbs, the sense and antisense transcripts showed complementary expression patterns, with antisense RNAs increasing in abundance in regions where sense RNAs were diminishing in abundance. Furthermore, targeted mutation of Hoxa 11 is shown to result in both male and female sterility. The female mutants produce normal ova, which develop properly post-fertilization when transferred to wild-type surrogate mothers. The Hoxa 11 homozygous mutants are shown to provide a defective uterine environment. The mutant males exhibited a malformation of the vas deferens that resembles a partial homeotic transformation to an epididymis. In addition, the mutant testes fail to descend properly into the scrotum and, likely as a result, spermatogenesis is perturbed.

Methionine deprivation regulates the translation of functionally-distinct c-Myc proteins

Numerous studies have demonstrated a critical role for the c-myc gene in the control of cellular growth. Alterations of the c-myc gene have been found associated with many different types of tumors in several species, including humans. The increased synthesis of one of the major forms of c-Myc protein, c-Myc 1, upon methionine deprivation provides a link between the regulation of oncogenes and the nutritional status of the cell. While deregulation or overexpression of the other major form, c-Myc 2, has been shown to cause tumorigenesis, the synthesis of c-Myc 1 protein is lost in many tumors. This suggests that the c-Myc 1 protein is necessary to keep the c-Myc 2 protein "in check" and prevent certain cells from becoming tumorigenic. Indeed, we have shown that overproduction of c-Myc 1 can inhibit cell growth. We have also shown that c-Myc 1 and 2 proteins have a differential molecular function in the regulation of transcription through a new binding site of Myc/Max heterodimers. We have also recently identified new translational forms of the c-Myc protein which we term delta-c-Myc. These proteins arise from translational initiation at downstream start sites which yield N-terminally-truncated c-Myc proteins. Since these proteins lack a significant portion of the transactivation domain of c-Myc, they behave as dominant-negative inhibitors of the full-length c-Myc 1 and 2 proteins. The synthesis of delta-c-Myc proteins is also regulated during cell growth and is repressed by methionine deprivation. Therefore, the synthesis of c-Myc 1 and delta-c-Myc proteins are reciprocally regulated by methionine availability. We have also found some tumor cell lines which synthesize high levels of the delta-c-Myc proteins. Taken together, our data suggest that c-Myc function is dependent on the levels of these different translational forms of c-Myc protein which are regulated by the nutritional status of the cell during growth. Numerous reports have demonstrated a fundamental and diverse role for the myc gene in cellular events, including proliferation, differentiation and apoptosis (Cole 1986; Spencer and Groudine 1991; Askew et al. 1991; Evan et al. 1992). This is dramatically illustrated by the frequent occurrence of a variety of tumors in many species having alterations of myc genes and the transduction of c-myc sequences by retroviruses (Spencer and Groudine 1991).4+ Eisenman 1990).(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation and function of non-AUG-initiated proto-oncogenes

A small, yet growing, number of cellular eukaryotic mRNAs encoding important regulatory proteins, such as c-myc and other proto-oncogenes, initiate translation from a non-AUG codon, usually in addition to initiating at a downstream AUG. The efficiency of non-AUG initiation on these natural cellular mRNAs varies considerably and appears to be governed by several features, including the codon sequence, the context surrounding the codon and the secondary structure of the transcript. In addition to factors which control the overall efficiency of c-myc non-AUG initiation, the relative efficiency of the upstream non-AUG initiation compared with the AUG initiation changes during the growth of cells. As lymphoid and fibroblast cells approach high densities in culture there is a sustained 5-10-fold induction in the synthesis of the non-AUG-initiated c-Myc 1 protein to levels comparable to or greater than the AUG-initiated c-Myc 2 protein. This increased efficiency of c-myc non-AUG initiation, due to methionine depletion of the growth medium, suggests that the scanning preinitiation complex can be regulated to enhance the recognition of a suboptimal non-AUG codon. The significance of non-AUG initiation for the growth-regulatory genes is illustrated by the different localizations of the int-2, bFGF and hck non-AUG-initiated proteins, the disruption of the c-myc and lyl-1 non-AUG initiation in tumor-derived cell lines, and the distinct biological function of the non-AUG-initiated forms of bFGF.