Biochemical analysis of MBD1

Abstract

Methylation of cytosines within CpG dinucleotides is a feature of vertebrate DNA. The precise role of DNA methylation is unknown to date, although it has been implicated in several processes relating to transcriptional regulation. One approach to study DNA methylation is the characterization of proteins that bind specifically to methylated DNA. One such family of proteins is the methyl-CpG binding domain (MBD) containing family and MBD1 is a member of this family. MBD1 is implicated in transcriptional repression and various mechanisms by which it might bring about gene silencing have been proposed. These are mainly based on studies reporting interactions between MBD1 and various proteins that regulate chromatin structure. Also MBD1 function can be modified by PIAS proteins, which stimulate its conjugation to SUMO (small ubiquitinlike modifier).The original aim of this work was to address two questions about MBD1: (1) Does MBD1 form part of a stable complex with other factors, and if so, what are the identities of the other components? Purification of MBD1 revealed the presence of no stably bound interacting proteins. However, some evidence indicates MBD1 may interact with itself and form dimers, a finding which impacts on many aspects of the function of MBD1. Also a proteomics screen for transient interaction partners identified candidate binding partners for MBD1 and the related protein MeCP2, which may throw light on the function of these proteins. (2) Are there any activities which regulate MBD1 function by the removal of SUMO from this protein? No activities capable of removing SUMO from native MBD1 were found but it was demonstrated that this modification leads to the destabilization of MBD1 in vitro. The relevance of this finding in vivo is yet to be determined.