The discs-large tumor suppressor gene of Drosophila encodes a guanylate kinase homolog localized at septate junctions.

Mutations of the lethal(1)discs large-1 (dlg) tumor suppressor gene of Drosophila cause neoplastic overgrowth of the imaginal discs. Sequencing of a near full-length cDNA predicts a protein containing a domain homologous to yeast guanylate kinase and a region homologous to SH3, a putative regulatory motif in nonreceptor protein tyrosine kinases and other signal transduction proteins. Immunofluorescence analysis using antibodies directed against fusion peptides shows that the dlg gene product is localized in an apical belt of the lateral cell membrane, at the position of the septate junction. The results suggest that a signal transduction process involving guanine nucleotides occurs at the septate junction and is necessary for cell proliferation control in Drosophila epithelia.