Evolution and pathophysiological structures of the N-terminal domain of MeCP2-E1

Methyl CpG-binding protein 2 (MeCP2) is a chromatin architectural protein that binds DNA CpG islands regulating transcription. Mutations in the MECP2 gene can cause Rett syndrome (RTT) a severe, X-linked, neurodevelopmental disease. MeCP2 is expressed in two isoforms, i.e. MeCP2-E1 and MeCP2-E2, and MeCP2-E1 is the most abundant in the central nervous system. Interestingly, unlike MeCP2-E2, MeCP2-E1 bears at the N-terminus two amino acid repeats (AARs), i.e. polyalanine (polyA) and polyglycine (polyG). Notably, single amino acid substitutions (A2V) and length polymorphisms of the MeCP2-E1 polyA and polyG repeats have been linked to RTT and other forms of intellectual disability. However, the pathophysiological structures and roles of these AARs are poorly understood. Here, we undertake an analysis of the evolution, structure, and interactions of the N-terminal domain of MeCP2 and its AARs, through a combination of bioinformatics, structural, and cell biological approaches. We find that polyA and other AARs in MeCP2 display distinctive patterns of occurrence, co-occurrence, and length variation in the evolution of vertebrates that are consistent with a functional regulatory role. Using circular dichroism, we find differential structural features and stability between the MeCP2-E1 and MeCP2-E2 N-termini, and between wild type MeCP2-E1 and disease-related polyA length variants. Finally, we explored the occurrence of polyA and polyG repeats among the MeCP2 interactors, and we studied the differential interaction capacity of the MeCP2-E1 and -E2 N-termini with known polyA-bearing MeCP2 interactors. Taken together, these findings support a relevant functional and pathological role for the AARs at the N-terminus of MeCP2-E1.