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MeCP2 is a 53 kDa nuclear protein named for its
ability to recognize methylated DNA. MeCP2 can act as a methylation-dependent
and -independent regulator of transcription in vitro and in vivo. MeCP2 also
is involved in the maintenance of condensed chromosomal superstructures, and
regulates mRNA splicing. MeCP2 is able to interact with many different
macromolecules and macromolecular complexes, including unmethylated and
methylated DNA, nucleosomes and chromatin, transcriptional co-repressors, a
histone H3 methyltransferase, Dnmt1 DNA methyltransferase, PU.1, and Y
box-binding protein 1 and other splicing factors. Although once thought of
strictly as a transcriptional repressor, MeCP2 is now recognized to be a
multifunctional nuclear protein with complex actions.
The importance of understanding MeCP2 structure/function relationships is
further underscored by its central role in the neurological disorder, Rett
Syndrome (RTT), an autism-spectrum X-linked neurodevelopmental disorder. RTT
is caused by nonsense, missense, and frameshift mutations found throughout
the entire MeCP2 gene (Mutation
Frequency) .
At this point there is no indication of why so many different mutations in
so many different locations are capable of leading to RTT, other than it
implies that all regions of the protein are functional.
Early on Adrian Bird’s group showed that MeCP2
has two well defined functional domains. Residues 78-162 are the minimal
sequences required to specifically recognize methylated CpG dinucleotides
and have been termed the methyl DNA binding domain (MBD). The minimal
sequence needed to repress transfected DNA has been called the
transcriptional repression domain (TRD) and consists of residues 207-310.
More recently, we characterized the tertiary structure of the protein using
analytical ultracentrifugation, circular dichroism, and protease digestion.
The surprising conclusion was that MeCP2 is composed of six
protease-resistant domains (see Fig. 1) that collectively are ~60-80%
unstructured, and are organized into a tertiary structure that has the
hydrodynamic properties of a random coil. Moreover, the disorder was
predicted to be present in all six domains, with many of the domain
boundaries corresponding to predicted order/disorder junctions (see Fig. 2).
These data established that MeCP2 is a unique intrinsically disordered
protein with a novel tertiary structure. Deciphering the unique structural
properties and features of MeCP2 and how they are related to function is one
of the major focuses of our research.
Chromatin architectural proteins are able to
condense nucleosomal arrays into novel higher order secondary and tertiary
chromatin structures in vitro in the absence of salts. We showed several
years ago that MeCP2 is such a protein (see Fig. 3) and we continue to study
in detail the mechanism of how MeCP2 condenses chromatin. The lab also
focuses on characterizing the unique tertiary structure of MeCP2, focusing
on the structural and functional relationships between order and disorder in
the protein sequence.
In both our protein chemistry and chromatin condensation studies, we compare
the behavior and properties of wild type MeCP2 with rationally designed
MeCP2 mutants to better understand the molecular basis for MeCP2 involvement
in normal cellular function and in RTT.
This project is funded by NIH grant R01 GM66834.
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