A distal dimerization domain is essential for DNA-binding by the atypical HNF1 homeodomain

Abstract

Hepatic Nuclear Factor 1 (HNF1, also referred to as LFB1, HP1 or APF) is a liver-specific transcription factor required for the expression of many hepatocyte specific genes. We report here the purification of this rat liver nuclear protein and the cloning of its cDNA using a PCR-derived approach. Seven independent clones reveal 3 alternative polyadenylation sites and a unique open reading frame. Both a motif homologous to the homeodomain and a distal dimerization domain are required for specific DNA binding. Sequence comparisons reveal several atypical features at key positions in the segment corresponding to helices III and IV of the Antaennapedia homeodomain as well as a potential 24 amino acid loop in place of the universal turn between helices II and III. Together with its property to dimerize in the presence or absence of DNA, these features place HNF1 as the prototype of a novel subclass of transcription factors distantly related to homeoproteins.

Expression profile of MODY3/HNF-1α protein in the developing mouse pancreas

“…HNF-1α staining was also detected in the stomach and intestine, but was not detected in the brain and heart. This restricted pattern of HNF-1α staining was compatible with the reported tissue distribution of HNF-1α mRNA [3,4,5,30,31] and β-galactosidase expression of HNF-1α (+/-) mice [6]. Furthermore, HNF-1α immunoreactivity was absent from the nuclei of hepatocytes in HNF-1α (-/-) mice (Fig.…”

“…Although HNF-1α was initially identified as a hepatocyte-specific transcription factor, it is known also to be expressed in the pancreas [3,4,5,6]. We have found that heterozygous mutations of the HNF-1α gene cause a form of maturity-onset diabetes of the young (MODY3) [7] and clinical studies have shown that MODY3 is characterized by impaired insulin secretion [8,9], suggesting an important role of HNF-1α in pancreatic beta cells.…”

HNF-1 alpha and HNF-1 beta (vHNF-1) share dimerization and homeo domains, but not activation domains, and form heterodimers in vitro.

“…Previous studies have shown that HNF-I~, as originally postulated ), binds to its recognition sequence as a dimer (Frain et al 1989;Chouard et al 1990;Nicosia et al 1990) and that this dimerization, which occurs in the absence of DNA, is dependent on the 30 amino acids at the extreme amino terminus of HNF-I~ (Chouard et al 1990;Nicosia et al 1990). The observation that the dimerization domain of HNF-loL is conserved in HNF-113, and that the dimerization domain is required for HNF-113 to bind DNA (Fig.…”

“…HNF-1 is unique among mammalian homeo domaincontaining proteins in that it contains a 21-amino-acid "loop" between helixes 2 and 3 of the homeo domain (Baumhueter et al 1990;Finney 1990) and it forms homodimers in solution (Chouard et al 1990;Nicosia et al 1990). The ability of HNF-1 to dimerize in solution, combined with the observation that a subset of the HNF-1-dependent genes are expressed in tissues and at various times in development, which appear inconsistent with the known expression pattern of HNF-1 (Baumhueter et al 1990), have raised the possibility that the function of the HNF-l-binding site could be diversified somehow developmentally either by the binding of other proteins or by developmental modifications of HNF-1.…”

Genetics of hepatocellular tumors

“…Hepatocyte nuclear factor 1a is a transcription factor that is implicated in hepatocyte differentiation and is required for the liver-specific expression of several genes, including b-fibrinogen, albumin and a1-antitrypsin (Frain et al, 1989;Baumhueter et al, 1990;Cereghini et al, 1990;Chouard et al, 1990). Heterozygous germline mutations of TCF1/HNF1a have been linked to the occurrence of MODY3 (maturity onset diabetes of the young, OMIM no.…”