FBPs Are Calibrated Molecular Tools To Adjust Gene Expression

Abstract

The three far-upstream element (FUSE) binding protein (FBP) family members have been ascribed different functions in gene regulation. They were therefore examined with various biochemical, molecular biological, and cell biological tests to evaluate whether their sequence differences reflect functional customization or neutral changes at unselected residues. Each FBP displayed a characteristic profile of intrinsic transcription activation and repression, binding with protein partners, and subcellular trafficking. Although some differences, such as weakened FBP3 nuclear localization, were predictable from primary sequence differences, the unexpected failure of FBP3 to bind the FBP-interacting repressor (FIR) was traced to seemingly conservative substitutions within a small patch of an N-terminal α-helix. The transactivation strength and the FIR-binding strength of the FBPs were in the opposite order. Despite their distinguishing features and differential activities, the FBPs traffic to shared subnuclear sites and regulate many common target genes, including c-myc. Though a variety of functions have been attributed to the FBPs, based upon their panel of shared and unique features, we propose that they constitute a molecular regulatory kit that tunes the expression of shared targets through a common mechanism.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank T. Misteli for helpful discussions and comments, L. Liotta and D. Braddock for critical review, and D. Libutti for preparing GST-FBP KH domain proteins. Imaging experiments were carried out in the Fluorescence Imaging Facility, LRBGE, NCI.

This work was supported by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research.