MafA has strong cell transforming ability but is a weak transactivator

Area of Science:

• Molecular Biology
• Oncology
• Virology

Background:

• The avian oncogenic retrovirus AS42 encodes the v-Maf oncoprotein, a nuclear bZip protein.
• Cellular Maf proteins (c-Maf, MafA, MafB) are related bZip proteins involved in gene regulation.
• The AP-1 transcription factor binding site is recognized by v-Maf.

Purpose of the Study:

• To compare the transactivation and cell transformation capabilities of MafA, MafB, and c-Maf proteins.
• To investigate the molecular mechanisms underlying Maf protein-induced cell transformation.
• To develop regulatable systems for studying Maf-mediated cellular responses.

Main Methods:

• Expression of MafA, MafB, and c-Maf proteins in chicken embryo fibroblasts.
• Reporter gene assays to measure transactivation activity.
• Construction of MafA-MafB chimeras and mutagenesis to identify functional domains.
• Phosphorylation site analysis of Maf proteins.
• Generation of MafA-estrogen receptor fusion proteins for hormone-dependent studies.

Main Results:

• All tested Maf proteins induced cellular transformation in chicken embryo fibroblasts.
• MafA exhibited significantly weaker transactivation activity compared to other Maf proteins.
• MafA demonstrated the strongest cell transformation activity, correlated with its DNA-binding domain.
• Phosphorylation of specific serine residues in MafA's transactivation domain influenced its activity.
• MafA-estrogen receptor fusion proteins displayed tightly hormone-dependent cell transformation.

Conclusions:

• The DNA-binding domain of MafA is critical for its potent cell-transforming ability.
• Both DNA-binding and phosphorylation events regulate MafA's transactivation and transformation functions.
• Regulatable MafA constructs enable detailed kinetic analysis of target gene responses and direct/indirect target discrimination.