Cytosolic Ras Supports Eye Development in Drosophila

Abstract

Ras proteins associate with cellular membranes as a consequence of a series of posttranslational modifications of a C-terminal CAAX sequence that include prenylation and are thought to be required for biological activity. In Drosophila melanogaster, Ras1 is required for eye development. We found that Drosophila Ras1 is inefficiently prenylated as a consequence of a lysine in the A₁ position of its CAAX sequence such that a significant pool remains soluble in the cytosol. We used mosaic analysis with a repressible cell marker (MARCM) to assess if various Ras1 transgenes could restore photoreceptor fate to eye disc cells that are null for Ras1. Surprisingly, we found that whereas Ras1 with an enhanced efficiency of membrane targeting could not rescue the Ras1 null phenotype, Ras1 that was not at all membrane targeted by virtue of a mutation of the CAAX cysteine was able to fully rescue eye development. In addition, constitutively active Ras1^12V,C186S not targeted to membranes produced a hypermorphic phenotype and stimulated mitogen-activated protein kinase (MAPK) signaling in S2 cells. We conclude that the membrane association of Drosophila Ras1 is not required for eye development.

We thank Denise Montell, Jessica Treisman, Marc Therrien, the Bloomington Drosophila Stock Center, and the Developmental Hybridoma Studies Bank for flies and antibodies. We also acknowledge FlyBase for critical information. We thank Laura Ekas and Emily Olson for technical help.

This work was supported by NIH grants GM055279, CA116034, and CA118495 to M.R.P. and GM085075-01A1 to E.A.B. as well as a Basil O'Connor Starter Scholar research award (grant no. 5-FY06) from the March of Dimes Foundation and the Hirschl Charitable Trust (to E.A.B.).