The Drosophila synaptotagmin-like protein bitesize is required for growth and has mRNA localization sequences within its open reading frame.

The vertebrate synaptotagmin-like protein granuphilin binds to the vesicle-trafficking proteins Rab27a and Munc18 and can modulate exocytosis of insulin-containing secretory granules in pancreatic beta cell lines. Here, we report the molecular and genetic characterization of bitesize, a granuphilin homolog and the only Drosophila synaptotagmin-like protein. Mutations that affect bitesize have reduced cell size and number, resulting in smaller animals that develop slowly. We also show that at least two classes of bitesize transcripts are localized to the apical plasma membrane in polarized epithelial cells. Whereas most cis-acting mRNA localization sequences map to 3' untranslated regions, bitesize contains a 2.2-kb sequence within its ORF that is necessary and sufficient for apical localization. Thus, we have found that bitesize is a metazoan example of a transcript for which all identifiable mRNA localization sequences are contained within the protein-coding region.