Structural Basis Of The Tirpartite Patj/Pals1/Mals2Complex Assembly

The assembly of supramolecular complexes in multidomain scaffold proteins iscrucial for cell polarity control. The asymmetric distribution of a set of evolutionarilyconserved cell polarity proteins or lipids is required for establishing and maintainingcell polarization.L27domain, initially identified in the Caenorhabditis elegans Lin-2and Lin-7proteins, exists in many scaffold proteins and involved in assembling essentialsupramolecular protein complexes that play critical roles in cell polarity. Study ofindividual L27domains revealed largely unfolded domains that require the formationof obligate heterodimers or heterotrimers to achieve well-folded structures, L27domains have not been found to form homo-oligomers. The primary role of L27domains is to form and maintain multiple protein complexes. The scaffold proteinPals1forms a complex with other two scaffold proteins, Patj and Mals2, through itstandem L27domains to regulate apical-basal polarity.In this thesis, we report the crystal structure of a4-L27-domain-containingheterotrimer. The L27domains of Patj (L27_Patj), Pals1（L27N and L27C） and Mals2(L27_Mals2) were covalently linked by two3C protease-cleavable segments to be asingle open reading frame. The heterotrimer consists of two cognate pairs ofheterodimeric L27domains with similar conformations. Structural analysis andbiochemical data further show that the dimers assemble noncooperatively.Additionally, such noncooperative assembly of two heterodimers can be observed inanother tripartite complex, DLG1/CASK/Mals2. This assembly mode is different withthe tandem L27domain of MPP7mediated tripartite complex assembly. Our resultsreveal a novel mechanism for tandem L27domain-mediated, supramolecular complexassembly with a non-synergistic mode.