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Molecular and Cellular Biology, July 2000, p. 5107-5118, Vol. 20, No. 14
Department of Tumor Genetics and Biology,
Kumamoto University School of Medicine, Kumamoto
860-0811,1 Division of Gastroenterology,
Department of Internal Medicine, Kyoto University Post-Graduate
School of Medicine, Sakyo-ku, Kyoto
606-8507,2 and Graduate School of
Biological Sciences, Nara Institute of Science and Technology,
8916-5, Takayama, Ikoma, Nara 630-0101,3 Japan
Received 21 December 1999/Returned for modification 10 February
2000/Accepted 24 April 2000
MBD1 is a mammalian protein that binds symmetrically methylated CpG
sequences and regulates gene expression in association with DNA
methylation. This protein possesses a conserved sequence, named
methyl-CpG binding domain (MBD), among a family of methyl-CpG binding
proteins that mediate the biological consequences of the methylation.
In addition, MBD1 has at least five isoforms due to alternative
splicing events, resulting in the presence of CXXC1, CXXC2, and CXXC3
in MBD1 isoforms v1 (MBD1v1) and MBD1v2, and CXXC1 and CXXC2 in MBD1v3
and -v4. In the present study, we have investigated the significance of
MBD, CXXC, and the C-terminal transcriptional repression domain (TRD)
in MBD1. A bacterially expressed MBD binds efficiently to densely
methylated rather than to sparsely methylated DNAs. In both
methylation-deficient Drosophila melanogaster SL2 cells and
mammalian CHO-K1 cells, MBD1v1 represses transcription preferentially
from both unmethylated and sparsely methylated promoters, while MBD1v3
inhibits densely methylated but not unmethylated promoter activities.
The CXXC3 sequence in MBD1v1 is responsible for the ability to bind
unmethylated promoter. Furthermore, we have constructed mutant-type
MBD1s in which the functionally important residues Arg22, Arg30, Asp32,
Tyr34, Arg44, Ser45, and Tyr52 are changed to alanine to investigate
the correlation between the structure and function of the MBD in MBD1.
Excepting those for Ser45 and Tyr52, none of the recombinant MBD
mutants bound to the densely methylated or unmethylated DNAs, and green fluorescent protein-fused MBD1 mutants did not localize properly in the
nucleus. All the MBD1v1 and -v3 mutants lost the activity of
methylation-dependent gene repression. Based on these findings we have
concluded that MBD1 acts as a transcriptional regulator depending on
the density of methyl-CpG pairs through the cooperation of MBD, CXXC,
and TRD sequences.
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Mechanism of Transcriptional Regulation by
Methyl-CpG Binding Protein MBD1
*
Corresponding author. Mailing address: Department of
Tumor Genetics and Biology, Kumamoto University School of Medicine,
2-2-1 Honjo, Kumamoto 860-0811, Japan. Phone: 81-96-373-5118. Fax:
81-96-373-5120. E-mail: mnakao{at}gpo.kumamoto-u.ac.jp.
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