Cloning and functional expression of an E box-binding protein from rat granulosa cells

Ovarian granulosa cells undergo cell growth and cytodifferentiation during follicular maturation. In a number of tissues, the gene expression that is responsible for the cytodifferentiation is largely dependent on E box(es) located upstream of the responsible genes. In this study, we report on the cloning of cDNA(s) encoding E box (5'-CACGTG-3')-binding protein from a rat granulosa cell cDNA library using a yeast one-hybrid system. When multiple E box sequences were used as target, we obtained a positive clone that encodes the rat homologue of upstream stimulatory factor 2 (USF2). An analysis of the nucleotide sequence and its deduced amino acid sequence reveals that rat USF2 protein consists of 346 amino acid residues and belongs to the basic helix-loop-helix/leucine zipper protein family. Northern blot analysis shows that rat USF2 mRNA exists as multiple forms between 1.6 and 2.2 kilobases. The size of the cloned insert was identical to that of the transcript of maximal length. Electrophoretic mobility shift assays showed that in vitro-translated rat USF2 specifically binds to the E box. In addition, cotransfection experiments with luciferase-reporter constructs in HepG2 cells reveal that the overexpression of rat USF2 leads to an increase of luciferase activity in the E box sequence-dependent manner. Thus, we report molecular cloning, expression, and functional characterization of full-length rat USF2 cDNA.