Multiple isoforms of the Drosophila Spatzle protein are encoded by alternatively spliced maternal mRNAs in the precellular blastoderm embryo

The spatzle gene is required for proper specification of positional informa tion along the dorsal-ventral axis of the Drosophila embryo and for inducti on of the innate immune response to fungal infection. It has been shown to encode a precursor of a Nerve Growth Factor-like ligand which is also a mem ber of the cys-knot protein superfamily. In dorsal-ventral patterning, the most widely accepted model of the pathway places Spatzle at the end of a ve ntrally restricted protease cascade that results in the proteolytic process ing of the precursor form of Spatzle to an active ligand which is thought t o bind to the Toll receptor. Here we show that the spatzle gene encodes at least ten different protein isoforms as a result of complex alternative spl icing in precellular blastoderm embryos. Multiple transcripts are clearly p resent up until the time of cellularization, at which point most transcript s can no longer be detected. Nine isoforms were expressed and at least five are efficiently secreted in a heterologous protein expression system. RNA microinjection experiments demonstrate that three isoforms completely rescu e embryos from spatzle null mothers, while most of the others rescue to a l esser extent. The phenotypic rescue activities of several isoforms and the relevance of these isoforms to the generation of the ventralizing signal ar e discussed.