| Asymmetric cell division is a widely used mechanism to generate cell diversity. Drosophila neuroblasts are an excellent system to study cell asymmetry: they establish distinct apical/basal cortical domains; have an asymmetric mitotic spindle aligned along the apical basal axis; and divide unequally to produce a large apical neuroblast and a small basal daughter cell (GMC). Basally localized proteins include Miranda and its cargo, the Prospero homeodomain protein. During mitosis, basal proteins are segregated into the GMC, where Prospero promotes GMC differentiation. At the apical cortex is an evolutionarily conserved Par3, Par-6, and atypical protein kinase C (aPKC) protein complex. The work in this thesis investigates mechanisms which establish neuroblast asymmetry.;Through a genetic analysis, I showed that Discs large (Dlg), Scribble (Scrib), and Lethal giant larvae (Lgl) tumor suppressor proteins act in a common pathway to regulate multiple aspects of neuroblast asymmetry. Dlg/Scrib/Lgl proteins show apical cortical enrichment at prophase/metaphase, followed by uniform cortical distribution. Mutants have defects in basal but not apical protein targeting, a reduced apical cortical domain, and reduced apical spindle size. Defects in apical cell and spindle pole size lead to symmetric or inverted neuroblast cell divisions. Inverted divisions correlate with the appearance of abnormally small neuroblasts and large GMCs, showing that neuroblast/GMC identity is more tightly linked to cortical determinants than cell size.;Through a deletion analysis, I characterized the Scrib functional domains. I found the Leucine-repeat-repeats (LRRs) are necessary for cortical targeting of Scrib, whereas the PDZ domains are required for efficient apical enrichment. The LRRs also direct Miranda to the cortex, yet the PDZs are necessary for efficient targeting of Miranda to the basal cortex. Lastly, I showed establishment of cell size asymmetry is dependent upon both the LRR and PDZ domains of Scrib.;I performed a mutational analysis to determine the role of aPKC in regulating cortical protein polarity, and the genetic interactions between aPKC and lgl. I found that aPKC is required to target a subset of proteins to the apical cortex, aPKC regulates basal protein targeting, and reduced aPKC activity suppresses most, but not all, lgl phenotypes.;This dissertation includes both my previously published and my co-authored materials. |
