The role of the small GTPase Rap in Drosophila R7 photoreceptor specification.

The Drosophila R7 photoreceptor provides an excellent model system with which to study how cells receive and "decode" signals that specify cell fate. R7 is specified by the combined actions of the receptor tyrosine kinase (RTK) and Notch (N) signaling pathways. These pathways interact in a complex manner that includes antagonistic effects on photoreceptor specification: RTK promotes the photoreceptor fate, whereas N inhibits. Although other photoreceptors are subject to only mild N activation, R7 experiences a high-level N signal. To counter this effect and to ensure that the cell is specified as a photoreceptor, a high RTK signal is transduced in the cell. Thus, there are two levels of RTK transduction in the photoreceptors: in R7 it is high, whereas in others it is low. Here, we address how this high-level RTK signal is transduced in R7 and find that, in addition to Ras, another small GTPase, Rap, is also engaged. Thus, when N activity is high, a robust RTK signal operates that uses both Ras and Rap, but when N activity is low, only a mild RTK signal is transduced and Ras alone suffices for the purpose.