Two highly related p66 proteins comprise a new family of potent transcriptional repressors interacting with MBD2 and MBD3.

Methyl-CpG-binding domain proteins (MBD) mediate functional responses of methylated DNA. MBD2 and MBD3 are components of the MeCP1 protein complex, which contains the Mi-2/NuRD complex and includes 66- and 68-kDa polypeptides. Here we identified two highly related 66-kDa proteins in a yeast two-hybrid screen with MBD2b. Based on the high degree of sequence conservation to the previously identified Xenopus p66 subunit of the Mi-2/NuRD complex, we termed these proteins hp66alpha and hp66beta. hp66alpha is the human orthologue of Xenopus p66, whereas hp66beta, previously identified as a component of the human MeCP1 complex, is a second member of a p66 gene family. Coprecipitation of hp66alpha and MBD2 demonstrates their in vivo association. Furthermore, confocal microscopy shows a nuclear colocalization of hp66alpha with hp66beta and MBD2 in a speckled pattern. hp66alpha is a potent transcriptional repressor reducing gene activity about 100-fold and is ubiquitously coexpressed with hp66beta in cell lines and in fetal and adult tissues. We demonstrate direct binding of both p66 family members to MBD2 as well as MBD3. Interestingly, hp66alpha, which binds with a higher affinity than hp66beta, interacts via two interaction domains in contrast to a single interaction domain present in hp66beta. These results demonstrate that two highly related mammalian p66 proteins display overlapping functions and are involved in methylation dependent transcriptional repression.