IDENTIFICATION AND CHARACTERIZATION OF STRATUM, A NOVEL REGULATOR OF POLARIZED BASEMENT MEMBRANE DEPOSITION IN DROSOPHILA MELANOGASTER

Abstract

Epithelia perform important biological roles in barrier formation, secretion, and absorption, and are composed of cells with distinct apical, lateral, and basal domains. Loss of epithelial cell polarity is associated with cancer metastasis. The basement membrane (BM), a thin layer of extracellular matrix secreted basally by epithelial cells, is implicated in the establishment of initial apical-basal polarity. However, little is known about how BM proteins themselves achieve a polarized distribution. Previously, our lab demonstrated that Crag, a guanine nucleotide exchange factor (GEF) that activates Rab10 GTPase, is essential for the restriction of major BM proteins to the basal surface. In addition, knockdown of Rab10 leads to mislocalization of the BM proteins. In this study, we identify Stratum, another putative Rab10 GEF, as a novel regulator of the asymmetric deposition of BM in the Drosophila follicular epithelium. Interestingly, we show that RNAi knockdown of stratum leads to mislocalization of BM to the apical surface, while not affecting the distribution of apical and basolateral polarity markers. We confirmed the BM phenotype by generating potential mutants for stratum, and showing that the human homolog of Stratum, Mss4, can rescue the BM mislocalization. These results indicate that Stratum is primarily involved in the polarized secretion of BM. Since Mss4 has been shown to exhibit GEF activity for Rab10, we tested whether Stratum may function with Rab10 in polarized BM deposition. We also showed that Stratum has a basal localization corresponding to that of Rab10, but so far are unable to determine whether Stratum is a GEF for Rab10. Further investigation is needed to elucidate the mechanism by which Stratum acts with other factors to control the polarized secretion of BM.