Molecular and Cellular Anatomy, University of Regensburg, Universitätsstr. 31, 93053, Regensburg, Germany.
Internal Medicine D, University Hospital of Münster, Domagkstr. 3a, 48149, Münster, Germany.
Cell Mol Life Sci. 2018 Sep;75(17):3269-3282. doi: 10.1007/s00018-018-2792-1. Epub 2018 Mar 10.
Apical-basal polarity is an important characteristic of epithelia and Drosophila neural stem cells. The conserved Par complex, which consists of the atypical protein kinase C and the scaffold proteins Baz and Par6, is a key player in the establishment of apical-basal cell polarity. Membrane recruitment of Baz has been reported to be accomplished by several mechanisms, which might function in redundancy, to ensure the correct localization of the complex. However, none of the described interactions was sufficient to displace the protein from the apical junctions. Here, we dissected the role of the oligomerization domain and the lipid-binding motif of Baz in vivo in the Drosophila embryo. We found that these domains function in redundancy to ensure the apical junctional localization of Baz: inactivation of only one domain is not sufficient to disrupt the function of Baz during apical-basal polarization of epithelial cells and neural stem cells. In contrast, mutation of both domains results in a strongly impaired protein stability and a phenotype characterized by embryonic lethality and an impaired apical-basal polarity in the embryonic epithelium and neural stem cells, resembling a baz-loss of function allele. Strikingly, the binding of Baz to the transmembrane proteins E-Cadherin, Echinoid, and Starry Night was not affected in this mutant protein. Our findings reveal a redundant function of the oligomerization and the lipid-binding domain, which is required for protein stability, correct subcellular localization, and apical-basal cell polarization.
顶端-基底极性是上皮细胞和果蝇神经干细胞的一个重要特征。保守的 Par 复合物由非典型蛋白激酶 C 和支架蛋白 Baz 和 Par6 组成，是建立顶端-基底细胞极性的关键因素。已经报道了几种机制可以实现 Baz 的膜募集，这些机制可能具有冗余功能，以确保复合物的正确定位。然而，描述的相互作用都不足以将蛋白质从顶端连接处置换。在这里，我们在果蝇胚胎中体内剖析了 Baz 的寡聚化结构域和脂质结合基序的作用。我们发现这些结构域在冗余中发挥作用，以确保 Baz 的顶端连接定位：只有一个结构域的失活不足以破坏 Baz 在上皮细胞和神经干细胞的顶端-基底极化过程中的功能。相比之下，两个结构域的突变会导致蛋白质稳定性严重受损，并表现出胚胎致死性和胚胎上皮和神经干细胞中顶端-基底极性受损的表型，类似于 baz 功能丧失等位基因。引人注目的是，这种突变蛋白与跨膜蛋白 E-Cadherin、Echinoid 和 Starry Night 的结合未受影响。我们的发现揭示了寡聚化和脂质结合结构域的冗余功能，这对于蛋白质稳定性、正确的亚细胞定位和顶端-基底细胞极化是必需的。
