Homeoproteins CDP and SATB1 interact: Potential for tissue-specific regulation

Homeoproteins are known to participate in development and cell type specifi cation. The homeoproteins CCAAT displacement protein (CDP) and special AT-r ich sequence binding protein 1 (SATB1) have been shown to bind to nuclear m atrix-associated regions and to act as repressors of many cellular genes. M oreover, binding of SATB1 to the mouse mammary tumor virus (MMTV) promoter region dramatically affects the tissue-specific transcription of this retro virus. Because protein-protein interactions are a common means of regulatin g homeoprotein function, we tested whether SATB1 and CDP interact in vivo a nd in vitro. SATB1 interacted with CDP through its DNA-binding domain, as d emonstrated by glutathione S-transferase (GST) pull-down assays. GST pull-d own assays also showed that CDP associated with SATB1 through three of its four DNA-binding domains (CR1, CR2, and the homeodomain). SATB1-specific an tisera, but not preimmune sera, precipitated CDP from nuclear extracts, and CDP-specific antisera precipitated SATB1 from the same extracts. Far-Weste rn blotting detected interaction of SATB1 and CDP in several different tiss ue extracts. Association of purified SATB1 and CDP in vitro resulted in the inability of each protein to bind to DNA in gel retardation assays. CDP ov erexpression in cultured T cells led to a loss of detectable SATB1 binding to the MMTV promoter region, as measured by gel shift experiments. CDP over expression also elevated MMTV long terminal repeat reporter gene activity i n transient-transfection assays, a result consistent with neutralization of the SATB1 repressor function in T cells. SATB1 is very abundant in certain tissues, particularly thymus, whereas CDP is relatively ubiquitous, except in certain terminally differentiated cell types. Because of the tissue and cell type distribution of SATB1 and CDP, we propose that the SATB1-to-CDP ratio in different tissues is a novel mechanism for homeoproteins to contro l gene expression and differentiation in mammals.