SMAR1 and Cux/CDP modulate chromatin and act as negative regulators of the TCRβ enhancer (Eβ)

Abstract

Chromatin modulation at various cis-acting elements is critical for V(D)J recombination during T and B cell development. MARβ, a matrix-associated region (MAR) located upstream of the T cell receptor β (TCRβ) enhancer (Eβ), serves a crucial role in silencing Eβ-mediated TCR activation. By DNaseI hypersensitivity assays, we show here that overexpression of the MAR binding proteins SMAR1 and Cux/CDP modulate the chromatin structure at MARβ. We further demonstrate that the silencer function of MARβ is mediated independently by SMAR1 and Cux/CDP as judged by their ability to repress Eβ-dependent reporter gene expression. Moreover, the repressor activity of SMAR1 is strongly enhanced in the presence of Cux/CDP. These two proteins physically interact with each other and colocalize within the perinuclear region through a SMAR1 domain required for repression. The repression domain of SMAR1 is separate from the MARβ binding domain and contains a nuclear localization signal and an arginine-serine (RS)-rich domain, characteristic of pre-mRNA splicing regulators. Our data suggest that at the double positive stage of T cell development, cis-acting MARβ elements recruit the strong negative regulators Cux and SMAR1 to control Eβ-mediated recombination and transcription.