| Research Abstract |
C-MYC has been suggested to be involved in cell proliferation, differentiation, transformation and apoptosis. We supposed that C-MYC plays such kaleidoscopic functions in partnership with various proteins. Besides the reported Max family proteins forming dimers with C-MYC via the basic helix-loop-helix leucine-zipper structure (bHLHZip) in the C-terminal region of C-MYC, we have screened proteins which interacts with C-MYC via the myc boxes, well-conserved among the myc family, in the N-terminal region. We applied the yeast two-hybrid system in the screening and have obtained several novel partner proteins including AMY-1 and MM-1 as well as reported proteins, CBF/NF-Y, ORC1, CDC6 and cdk inhibitor p21. MSSP, which we have also identified as a C-MYC binding protein, recognized the myc boxes. These proteins were examined for their functions both in vitro and in vivo and were classified to the categories including factors for transcription (AMY-1, MM-1 and CBF/NF-Y), DNA replication (MSS
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P, ORC1 and CDC6) and cell-cycle movement (p21). As for the transcriptional activity of C-MYC, only AMY-1 acted as a positive regulatory factor while the others, MM-1 and CBF/NF-Y, were negative factors. AMY-1 induced the erythrocyte differentiation of K562 cells. Male transgenic mice of the AMY-1 gene were sterile and the RII domain of AKAP-84, the binding site of the regulatory subunit of A-kinase which is important in spermatogenesis, was cloned as a AMY-1 binding protein. AMY-1 was thus suggested to inhibit the appropriate localization of the A-kinase and to induce apoptosis of spermatogenic cells. In addition, AMY-1 was found to bind to WAVE/AKAP-149, a factor involved in actin polimerization. MM-1 has a character for putatice tumor suppressor. The substitution of the amino acid #157 of MM-1 from alanine to arginine was frequently observed in the cells and tissues of lymphoma, leukemia and tongue cancer. The arginine mutation abrogated the negative effects of MM-1 on C-MYC. ORC1, a remodeling factor of chromatin, bound to C-MYC competitively with SNF5, an MSSP family protein, to release C-MYC from chromatin. Null-mutation of MSSP tended to be lethal during developmental stage. These findings altogether suggest that C-MYC binding proteins play roles at crucial steps of cell maturation, fertilazation in addition to the modulation of versatile functions of C-MYC. Less
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