Spatial Regulation of the Polarity Protein aPKC During Asymmetric Cell Division of Drosophila Neuroblasts
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The Par complex protein, atypical protein kinase C (aPKC), plays an instrumental role in diverse cell polarities. aPKC is able to restrict substrate localization through a phosphorylation-induced cortical exclusion mechanism, allowing for the generation of molecularly distinct cortical domains. Thus, controlling the localization of aPKC is central to Par-mediated polarity but the mechanism by which aPKC is polarized remains poorly understood. In this dissertation I investigated the restriction of aPKC to the apical cortex of Drosophila neural stem cells, neuroblasts, as these cells dynamically polarize aPKC through repeated asymmetric cell divisions. The polarity created through aPKC phosphorylation must be tightly regulated in order to ensure proper balance between self-renewal and differentiation. To begin, I investigated whether or not aPKC’s so called ‘maturation’ by PDK1 phosphorylation is required for aPKC activity and localization. We found that aPKC’s phosphorylation by PDK1 is required for both polarity and full activity. An aPKC containing an unphosphorylatable activation loop mutation localizes symmetrically around the cortex in a manner independent of its binding partner, Par-6, suggesting that aPKC could interact with the cortex by an unknown mechanism. To investigate how aPKC is able to localize to the cortex independent of Par-6, I used an in vivo structure function analysis of domains within aPKC, accompanied by biochemical approaches. I identified a necessity for the aPKC C1 domain for binding to the neuroblast cortex. This interaction is mediated by negatively charged phospholipids. Neither aPKC interaction, with phospholipids or Par-6, is sufficient to restrict aPKC to the apical cortex. Thus, aPKC polarization utilizes a dual interaction mechanism that takes advantage of both protein-lipid and protein-protein interactions, and proper control of each of these signals is required to prevent neuroblast division defects. One interaction, mediated by the C1, is a general cortical targeting mechanism, whereas the other specifies polarization mediated by Par complex interactions. We conclude that a conformational change induced by these interactions activates aPKC’s catalytic activity, thereby coupling localization and activity. This dissertation includes unpublished co-authored material.