Drosophila unpaired encodes a secreted protein that activates the JAK signaling pathway

Abstract

In vertebrates, many cytokines and growth factors have been identified as activators of the JAK/STAT signaling pathway. In Drosophila, JAK and STAT molecules have been isolated, but no ligands or receptors capable of activating the pathway have been described. We have characterized the unpaired (upd) gene, which displays the same distinctive embryonic mutant defects as mutations in the Drosophila JAK (hopscotch) and STAT (stat92E) genes. Upd is a secreted protein, associated with the extracellular matrix, that activates the JAK pathway. We propose that Upd is a ligand that relies on JAK signaling to stimulate transcription of pair-rule genes in a segmentally restricted manner in the early Drosophila embryo.

The roles of the Drosophila JAK/STAT pathway

“…Biochemical analysis of Upd revealed that the protein has an apparent molecular weight of 65 kDa, is indeed glycosylated and is localized in the extracellular matrix. Furthermore, tissue culture experi- (Harrison et al, 1998). Taken together with in vivo evidence showing that Upd functions as an activator of JAK/STAT signaling in the eye (Zeidler et al, 1999b), it is clear that Unpaired represents a bona ®de JAK/STAT pathway ligand.…”

“…With the recent cloning of unpaired (upd) and its identi®cation as a potential ligand for the Drosophila JAK/STAT pathway (Harrison et al, 1998), three pathway components have been identi®ed on the basis of their biochemical, genetic and mutant phenotypes.…”

“…While mutations in the unpaired locus have been known for over 75 years, only recently has the associated gene been identi®ed as a ligand for the Drosophila JAK/STAT pathway (Venderosa and Muller, 1954;Harrison et al, 1998). The unpaired locus encodes a predicted 47 kDa secreted protein with an N-terminal signal sequence and several potential Nlinked glycosylation sites (Figure 1b).…”

“…The unpaired locus encodes a predicted 47 kDa secreted protein with an N-terminal signal sequence and several potential Nlinked glycosylation sites (Figure 1b). While a protein similar to Upd called Om1E has been identi®ed in the closely related fruit¯y Drosophila ananassae, no known vertebrate homologues have been found in publicly available databases to date (Harrison et al, 1998;Juni et al, 1996). Biochemical analysis of Upd revealed that the protein has an apparent molecular weight of 65 kDa, is indeed glycosylated and is localized in the extracellular matrix.…”

“…Hop is expressed essentially uniformly throughout the various stages of embryonic development, while Stat92E is expressed uniformly during early stages but subsequently resolves initially into seven and then 14 segmental stripes (Binari and Perrimon, 1994;Hou et al, 1996;Yan et al, 1996a). Upd, in contrast, is not maternally supplied but is expressed from the zygotic genome in a highly dynamic pattern: broadly in the trunk and in a stripe in the head before cellularization, seven stripes during cellularization, 14 stripes during gastrulation and later in the tracheal pits (Harrison et al, 1998). Despite these very di erent patterns of expression the three known components of the JAK/ STAT pathway exhibit very similar, loss-of-function phenotypes in the embryo.…”