Sec61 beta, a subunit of the protein translocation channel, is required during Drosophila development

We have identified and isolated mutations in the first Drosophila gene enco ding a subunit of the Sec61 protein translocation channel, DSec61 beta, Whi le neither the Saccharomyces cerevisiae Sec61 beta nor its functional Esche richia coli homologue are essential for viability or for protein translocat ion, we show that DSec61 beta is essential for embryonic development, Homoz ygous mutant embryos die at the end of embryogenesis and are impaired in th e secretion of cuticle proteins from the epidermis, DSec61 beta germ line c lones, result in defects in dorso-ventral patterning of the egg and are con sistent with affected secretion of the protein Gurken from the oocyte to th e follicle cells, Clonal analyses in the imaginal discs reveal defects in a dult structures, including rhabdomere morphogenesis and a reduction of the size of tarsal segments in the leg, This is the first in vivo study of a co mponent of the protein translocation machinery in higher eukaryotes, and il lustrates how a protein that has an inessential, kinetic function in single -cell organisms can become critical for the complex development of a multic ellular organism.