Identifying new regulators of secretory capacity. Rebecca M. Fox, Xueni Chen, Deborah J. Andrew. Dept Cell Biol, Johns Hopkins Univ, Baltimore, MD.

   In the Drosophila embryo, the salivary gland is the largest secretory organ, making it an excellent system in which to study the acquisition of high-level secretory function. We have shown previously that the bZip transcription factor, CrebA, and its mammalian orthologues, Creb3L1 and Creb3L2, are required to boost high-level secretory capacity through the direct regulation of components of the secretory pathway machinery. To identify other regulators of secretory function we performed microarray analyses to find the full complement of CrebA target genes. Of the nearly 400 genes whose expression went down in CrebA mutants, we chose to focus on genes encoding potential transcriptional regulators with clear human orthologues. Our initial analysis included whole mount in situs to look at the full embryonic expression patterns of each gene, as well as assaying for potential secretory defects in P-element or deficiency lines. Secretory defects present as defects in the cuticle secreted by the epithelial cells or as irregularities in the salivary gland or salivary gland lumen. Of the sixteen genes we characterized, Tudor-SN emerged as a promising potential regulator of secretory function based on this initial analysis. The Tudor-SN open reading frame is highly conserved and contains a Tudor DNA/RNA binding domain and four staphylococcal nuclease domains. Moreover, loss of Tudor-SN resulted in cuticle and salivary gland defects similar to those seen with loss of CrebA. Tudor-SN orthologues in other organisms have been implicated in multiple seemingly unrelated processes including transcriptional activation, mRNA stabilization, small RNA processing, stress granule formation, lipoprotein phospholipid secretion and spliceosome assembly. Our initial experiments in Drosophila have confirmed that Tudor-SN is regulated by CrebA and have shown that the protein localizes to the ER. We expect that the characterization of CrebA targets, such as Tudor-SN, will provide key insights into the regulation and implementation of secretory function in p.