Myb oncogene-like: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - Myb oncogene-like

Synonyms - Dm-Myb, c-Myb

Cytological map position - 13F2--13F2

Function - transcription factor

Keywords - cell cycle, oncogene

Symbol - Myb

FlyBase ID: FBgn0002914

Genetic map position - 1-[50]

Classification - Myb DNA-binding domain

Cellular location - nuclear

NCBI links: | Entrez Gene

The proto-oncogene c-myb was first encountered in chickens as a transduced retroviral oncogene, v-myb, which causes a myeloid leukemia and transforms myeloid cells in culture. Mutations affecting c-myb have since been implicated in the tumors of both humans and mice. The product of c-myb (Myb) is a transcription factor that binds a specific DNA sequence. The protein is divided into at least three discrete functional domains: one for binding to DNA, a second for activation of transcription, and a third domain that governs the biochemical activity of the protein. It is generally believed that Myb proteins, including MybA and MybB, two additional vertebrate Myb proteins that are related to c-myb, play roles in the cell division cycle. c-myb and MybB have been implicated in the G1/S transition, whereas MybA is more likely to be involved in cellular differentiaton. Myb-like proteins with related DNA-binding domains have also been found in yeast and plants (Katzen, 1998 and references).

Drosophila Myb oncogene-like is required for both mitosis and prevention of endoreduplication in wing cells. Drosophila Myb apparently acts at or near the time of the G2/M transition. The two mutant alleles of Drosophila Myb produce the same morphologically altered phenotype, although the responsible mutations are located in different functional domains of the gene product. Mutant wings have approximately half the number of hairs as wild-type wings, and the mutant hairs are considerably larger than normal. Mutant hairs are less regularly spaced, less uniform in orientation, and occasionally grouped in small clusters, indicating a disturbance of tissue polarity. The mutant phenotype can be partially suppressed by ectopic expression of either cdc2 or string, two genes that are known to promote the transition from G2 to M. Drosophila Myb is thought to be required for completion of cell division and may serve two independent functions: (1) promotion of mitosis and (2) prevention of endoreduplication when cells are arrested in G2 (Katzen, 1998).

In wild-type wings, each cell that is not specialized for another purpose is represented by a single hair. At 3 hr after puparium formation (APF) all wing cells except those at the anterior and posterior margins arrest synchronously in G2. The arrest persists until 12 hr APF, when mitosis occurs, followed by an additional cell division cycle that is completed by 24 hr APF. Thereafter, the cells remain in G0. For wings dissected out of mutant pupae between 24 and 36 hr APF, the density of nuclei in any given region of the wing is approximately half of that found for the same region of a wild-type wing, and the mutant nuclei are larger than their wild-type counterparts. Since no cell death is observed in mutant wings, it is concluded that mutant wings have fewer but larger cells, each of which produces a hair. This result points to a defect in cellular proliferation during the final stages of wing development in the Drosophila Myb mutants. When examined during the early G2 arrest (at 6 hr APF), the number of wing cells in myb1 mutants is normal, but in postmitotic wings (after 24 hr APF), the number of wing cells in myb1 mutants is half that of wild type. It is not possible to examine the wings between 7 and 24 hr APF, the period during which the final cell cycles occur. If both of the cell cycles that normally ensue are defective, the number of cells should be reduced by a factor of four, not just two. Thus, it is likely that only the final cell division fails to occur in the mutant wings (Katzen, 1998).

BrdU incorporation studies show that mutant as well as wild-type wings actively replicating DNA. The patterns of BrdU incorporation for the myb1 wings indicate that they are less mature than the wild-type wing, a finding that is consistent with observations that myb1 mutants develop more slowly than the parental flies. It is concluded that myb1 mutant wing cells enter into their final S phase but do not undergo a final division, presumably leaving them with a 4C content of DNA. To assess further the cell cycle state of the mutant Myb wing cells, a test was performed to determine whether the mutant phenotype can be suppressed by overexpression of either wild-type or activated alleles of cdc2 (the cyclin-dependent kinase that regulates the G2/M transition) or string (the Drosophila homolog of cdc25, the protein tyrosine/threonine phosphatase that regulates cdc2 activity). Overexpression of either cdc2 or string is able to partially suppress the myb2 wing phenotype, increasing the number of hairs by 30% to 35% and correcting the orientation of the hairs. These results support the conclusion that the mutant Myb wing cells are arrested at the G2/M transition (Katzen, 1998).

High-resolution, three-dimensional wide-field fluorescence microscopy was used to examine the amount of DNA contained in nuclei of individual wild-type and mutant cells. The average DNA content of mutant myb1 nuclei proves to be double that of wild-type nuclei, whereas the average for myb2 nuclei is only slightly elevated. Surprisingly, the average DNA content of mutant myb1 nuclei is even greater at 25°C than at 18°C, and the average for myb2 nuclei increases to approximately double that of wild-type. The mutant cells generally contain more DNA than wild-type cells. But there is substantial heterogeneity in the data for mutant cells, first apparent as relatively large standard deviations for the measurements of DNA content. Analysis of the data for individual cells provides an explanation: at the semi-permissive temperature of 18°C, the amount of DNA in myb1 nuclei varies from 2C to well in excess of 4C. At 25°C, the majority of nuclei contain more than 4C DNA. It is concluded that the mutation causes endoreduplication in a fraction of the arrested wing cells, the severity of which varies from one cell to another (Katzen, 1998).

These studies conclude that two cell cycle checkpoints appear to be disturbed in the wing cells of Myb mutants: regulation of the G2/M transition and prevention of re-entry into S phase before M phase occurs. Is one of these defects a consequence of the other or are they independent of each other? In string mutant embryos, cells arrest at the G2/M boundary and do not enter S phase, indicating that the mechanism for preventing reinitiation of DNA replication before mitosis can remain intact when cells are prematurely arrested in G2. Therefore, it is unlikely that the endoreduplication in mutant Myb cells is simply a consequence of the abnormal arrest in G2. Because overexpression of either cdc2 or string can partially suppress the Myb phenotype, at least a proportion of the mutant cells must still be competent for mitosis, suggesting that the block in G2 is not just a consequence of the mutant cells entering into an endocycle and losing the ability to divide. It is concluded that Myb may play active and independent roles in both the G2/M checkpoint and the S phase reduplication checkpoint. Final determination of whether Myb function is required in all cell cycles will have to await the generation of stronger alleles of Myb (Katzen, 1998).


Amino Acids - 657

Structural Domains

The four regions of conservation shared between vertebrate and Drosophila Myb proteins consist of three imperfect tandem repeats (R1, R2, and R3) that comprise the DNA-binding domain (region I), a transcriptional activator domain (TA - region II), a leucine zipper (LZ - region III), and a negative regulatory domain (NR - region IV). An additional region of the mouse protein, encoded by an alternatively spliced exon, contains the majority of conserved region II. The DNA-binding domain contains the Drosophila Myb2 mutation. In this region, chicken and human sequences are identical to the mouse sequence. Region IV includes the Drosophila Myb1 mutation (Katzen, 1998)

The proto-oncogene c-Myb from Drosophila melanogaster is represented by a single locus at position 13E-F on the X chromosome, and is expressed in early embryos by transcription into two polyadenylated RNAs with lengths of approximately 3.0 and 3.8 kb. The gene may encode a protein with a molecular weight of at least 55,000 that shares a domain with c-myb (chicken) in which 91 of 125 (or 73%) of the amino acids are identical in the Drosophila and chicken genes. These findings represent the first rigorous identification of a Drosophila proto-oncogene that can encode what may be a nuclear protein; they set the stage for a genetic analysis of how Myb serves the normal organism (Katzen, 1985).

The Drosophila melanogaster homolog of c-myb contains two clusters of sequences homologous to vertebrate myb proteins, surrounded by sequences lacking homology. These results extend previous evidence for the existence of a D. melanogaster homolog of c-myb and identify two highly conserved and therefore presumably functionally important domains of c-myb proteins. DNA-binding experiments indicate that the NH2-proximal of the two homology regions functions as a DNA-binding domain. Based on the absence of the COOH-proximal homology region in truncated oncogenic derivatives of c-myb, it is likely that this homology region encodes a function whose loss is involved in activating the oncogenic potential of c-myb (Peters, 1987).

Myb oncogene-like: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 21 April 98

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