Publications

Characteristically, an individual member of the superfamily of G protein-coupled receptors can interact only with a limited number of the many structurally closely related G protein heterotrimers that are expressed within a cell. Interestingly, the N termini of two G protein alpha subunits, Galphaq and Galpha11, differ from those of other alpha subunits in that they display a unique, highly conserved six-amino acid extension. To test the hypothesis that this sequence element is critical for proper receptor recognition, we prepared a Galphaq deletion mutant (-6q) lacking these first six amino acids. The -6q construct (or wild type Galphaq as a control) was coexpressed (in COS-7 cells) with several different Gi/o- or Gs-coupled receptors, and ligand-induced increases in inositol phosphate production were determined as a measure of G protein activation. Whereas these receptors did not efficiently interact with wild type Galphaq, most of them gained the ability to productively couple to -6q. Additional experiments indicated that the observed functional promiscuity of -6q is not due to overexpression (as compared with wild type Galphaq) or to a lack of palmitoylation. We conclude that the N-terminal extension characteristic for Galphaq/11 proteins is critical for constraining the receptor coupling selectivity of these subunits, indicative of a novel mechanism by which the fidelity of receptor-G protein interactions can be regulated.

The genes encoding the functionally related hepatocyte nuclear factors HNF-1alpha and HNF-4alpha play a critical role in normal pancreatic beta-cell function. Mutations in these liver-enriched transcription factors result in two forms of early-onset type 2 diabetes (maturity-onset diabetes of the young [MODY]), MODY3 and MODY1, which are characterized by impaired glucose-stimulated insulin secretion, early disease onset, and autosomal dominant inheritance. The transcriptional hierarchy of HNFs suggests that other proteins of the regulatory cascade might be responsible for other forms of MODY and/or late-onset type 2 diabetes. In this study, we show that HNF-3alpha, -3beta, -3gamma, -4gamma, and -6 are expressed in pancreatic beta-cells. We report the identification and characterization of simple tandem repeat DNA polymorphisms in the genes encoding HNF-3alpha, -3beta, -3gamma, -4gamma, and -6 and the mapping of HNF-6 to chromosome bands 15q21.1-21.2 by fluorescence in situ hybridization. These markers will be useful to study the role of genetic variation in these genes in the pathogenesis of type 2 diabetes.