Editors' ChoiceDevelopment

Torsos and Truncated Trunks

Science's STKE  10 Apr 2001:
Vol. 2001, Issue 77, pp. tw1
DOI: 10.1126/stke.2001.77.tw1

The transmembrane-spanning receptor tyrosine kinase Torso (Tor) is evenly dispersed along the entire surface of the developing Drosophila embryo; however, Tor is activated only at the embryonic poles, suggesting that Tor's ligand is either produced and secreted at the poles, or becomes an active ligand only at the poles. Casali and Casanova hypothesized that, on the basis of its amino acid sequence, Trunk (Trk), a protein important for Tor signaling but whose function is unknown, might undergo proteolytic cleavage. Previous reports indicated that mutations near the likely cleavage site rendered Trk inactive. Expression of a COOH-terminal Trk fragment (TrkC-108) in Drosophila embryos activated Tor in the absence of other gene products required for proper Tor activation, indicating that Trk function lies downstream of those gene products, and that Trk processing may be required for Tor activation. TrkC-108 could not rescue the development of a Tor-deficient Drosophila mutant, suggesting that Trk might be a ligand for Tor. The authors speculate that inactive Trk is secreted but is only activated when cleaved by a proteolytic enzyme complex located at the poles proximal to Tor. Furthermore, Casali and Casanova suggest that some of the other gene products required for the activation of Tor through Trk contain protease activity.

A. Casali, J. Casanova, The spatial control of Torso RTK activation: A C-terminal fragment of the Trunk protein acts as a signal for Torso receptor in the Drosophila embryo. Development 128, 1709-1715 (2001). [Online Journal]