The human proteins MBD5 and MBD6 associate with heterochromatin but they do not bind methylated DNA.
BACKGROUND
MBD5 and MBD6 are two uncharacterized mammalian proteins that contain a putative Methyl-Binding Domain (MBD). In the proteins MBD1, MBD2, MBD4, and MeCP2, this domain allows the specific recognition of DNA containing methylated cytosine; as a consequence, the proteins serve as interpreters of DNA methylation, an essential epigenetic mark. It is unknown whether MBD5 or MBD6 also bind methylated DNA; this question has interest for basic research, but also practical consequences for human health, as MBD5 deletions are the likely cause of certain cases of mental retardation.


RESULTS
Here we report the first functional characterization of MBD5 and MBD6. We have observed that the proteins colocalize with heterochromatin in cultured cells, and that this localization requires the integrity of their MBD. However, heterochromatic localization is maintained in cells with severely decreased levels of DNA methylation. In vitro, neither MBD5 nor MBD6 binds any of the methylated sequences DNA that were tested.


CONCLUSIONS
Our data suggest that MBD5 and MBD6 are unlikely to be methyl-binding proteins, yet they may contribute to the formation or function of heterochromatin. One isoform of MBD5 is highly expressed in oocytes, which suggests a possible role in epigenetic reprogramming after fertilization.
Laget
Sophie
S
New England Biolabs, Ipswich, Massachusetts, United States of America.
Joulie
Michael
M
Le Masson
Florent
F
Sasai
Nobuhiro
N
Christians
Elisabeth
E
Pradhan
Sriharsa
S
Roberts
Richard J
RJ
Defossez
Pierre-Antoine
PA
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D004247:DNA / Q000378:metabolism*; D019175:DNA Methylation; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation; D006570:Heterochromatin / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D009928:Organ Specificity; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D021381:Protein Transport
D004268:DNA-Binding Proteins; D006570:Heterochromatin; C550332:MBD5 protein, human; C553236:MBD6 protein, human; D020033:Protein Isoforms; D004247:DNA