Frizzled-suc2 fusion gene studies in Saccharomyces cerevisiae.

The frizzled (fz) gene is required for development of planar tissue polarity in the epidermis of Drosophila melanogaster; it likely encodes an integral membrane protein that functions as a receptor for a tissue polarity-signaling molecule. On the basis of hydropathy analyses, it has been postulated that Fz protein contains seven transmembrane domains. Consistent with that model, the amino terminus of Fz was found to be extracellular, whereas the carboxyl terminus was intracellular. In the present study, the membrane topology of Fz was further examined in vivo by constructing various fz-suc2 fusion genes and analyzing their invertase activity in transformed yeast. We observed that plating efficiency and growth rate on sucrose media, as well as invertase secretion, were consistent with the proposed seven-pass model of Fz. This study provides the first experimental assessment of overall Fz topology.