Homodimerization of Ldb1 forms the basis for long-range enhancer looping of the beta-globin LCR in vivo

Thursday, October 11, 2012 — Poster Session III
10:00 a.m. – Noon	Natcher Conference Center, Building 45	NIDDK	GEN/GENOM-6

* FARE Award Winner

Authors

• I Krivega
• A Dean

Abstract

Long-range interaction between the beta-globin LCR enhancer and gene involves a multi-protein complex that includes Ldb1, LMO2, GATA-1 and TAL1. Ldb1 is required for long-range chromatin looping and beta-globin gene activation. Ldb1 can homodimerize in vitro through its N-terminal dimerization domain (DD). We tested whether the homodimerization of Ldb1 plays a key role in LCR/beta-globin looping and transcription activation in erythroid cells. Full-length, shRNA-immune Ldb1, rescued long-range LCR/beta-globin interaction and beta-globin gene expression in Ldb1 knock-down MEL cells. Ldb1 proteins with deletions of short conserved sequences throughout the DD domain expressed in the background of Ldb1 KD MEL cells were unable to rescue beta-globin expression. Co-immunoprecipitation experiments using wild type MEL cells expressing DD confirmed that DD homodimerize with endogenous Ldb1 and, through it, interacts with other members of the Ldb1 complex. DD domains with deletions of short conserved sequences were unable to homodimerize with endogenous Ldb1. Finally, a fusion protein of DD with LMO2 (LMO-DD) was expressed in Ldb1 KD MEL cells. LMO-DD rescued long-range LCR/beta-globin interaction and beta-globin gene expression. These results support the hypothesis that homodimerization of Ldb1 provides the link between LCR and beta-globin bound complexes that mediate chromatin looping and gene activation.

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