Membrane Targeting and Asymmetric Localization of Drosophila Partner of Inscuteable Are Discrete Steps Controlled by Distinct Regions of the Protein

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Abstract
Asymmetric division of neural progenitors is a key mechanism by which neuronal diversity in the Drosophila central nervous system is generated. The distinct fates of the daughter cells derived from these divisions are achieved through preferential segregation of the cell fate determinants Prospero and Numb to one of the two daughters. This is achieved by coordinating apical and basal mitotic spindle orientation with the basal cortical localization of the cell fate determinants during mitosis. A complex of apically localized proteins, including Inscuteable (Insc), Partner of Inscuteable (Pins), Bazooka (Baz), DmPar-6, DaPKC, and G alpha i, is required to mediate and coordinate basal protein localization with mitotic spindle orientation. Pins, a molecule which directly interacts with Insc, is a key component required for the integrity of this complex; in the absence of Pins, other components become mislocalized or destabilized, and basal protein localization and mitotic spindle orientation are defective. Here we define the functional domains of Pins. We show that the C-terminal region containing the G alpha i binding GoLoco motifs is necessary and sufficient for targeting to the neuroblast cortex, which appears to be a prerequisite for apical localization of Pins. The N-terminal tetratricopeptide repeat-containing region of Pins is required for two processes; TPR repeats 1 to 3 plus the C-terminal region are required for apical localization but are insufficient to recruit Insc to the apical cortex, whereas TPR repeats 1 to 7 plus C-terminal Pins can perform both functions. Hence, the abilities of Pins to cortically localize, to apically localize, and to restore Insc apical localization are all separable, and all three capabilities are necessary to mediate asymmetric division. Moreover, the need for N-terminal Pins can be obviated by fusing a minimal Insc functional domain with the C-terminal region of Pins; this chimeric molecule is apically localized and can fulfill the functions of both Insc and Pins.