Stratum is required for both apical and basolateral transport through stable expression of Rab10 and Rab35 in Drosophila photoreceptors.

Post-Golgi transport for specific membrane domains, also termed polarized transport, is essential for the construction and maintenance of polarized cells. Highly polarized Drosophila photoreceptors serve as a good model system for studying the mechanisms underlying polarized transport. The Mss4 Drosophila ortholog, Stratum (Strat), controls basal restriction of basement membrane proteins in follicle cells, and Rab8 acts downstream of Strat. We investigated the function of Strat in fly photoreceptors and found that polarized transport in both the basolateral and the rhabdomere membrane domains was inhibited in Strat -deficient photoreceptors. We also observed 79 and 55% reductions in Rab10 and Rab35 levels, respectively, but no reduction in Rab11 levels in whole-eye homozygous clones of Stratnull . Moreover, Rab35 was localized in the rhabdomere, and loss of Rab35 resulted in impaired Rh1 transport to the rhabdomere. These results indicate that Strat is essential for the stable expression of Rab10 and Rab35, which regulate basolateral and rhabdomere transport, respectively, in fly photoreceptors.