br Genetic analysis of polarity complexes Model organisms su

Genetic analysis of polarity complexes Model organisms such as Caenorhabditis elegans and Drosophila melanogaster provide useful means of describing basic biological processes. They lend themselves particularly well to addressing some of the questions arising in this field because of their structural simplicity and stereotyped features. In addition, their genomes have been completely sequenced and large-scale genetic screens are easy to perform. Genetic studies have shown the existence of evolutionarily conserved and ubiquitously expressed proteins that regulate cell polarization in many different cellular contexts (Table 1).
Polarity complex composition and interactions
Functional interactions between complexes and activation In this part, we will review how the different polarity complexes cooperate or interact together. It has by now been clearly established that the three complexes are connected through several protein–protein interactions. Genetic studies on flies have shown that mutants for Scrib complex show opposite phenotypes to those observed for mutants of members of the Crb and Par complexes [39], which suggests that the basolateral SCRIB complex and the apical CRB and PAR complexes have antagonistic activities. A connection between these different complexes was shown using genetic studies on flies and biochemical studies on mammalian NSC 207895 [39], [158] (Fig. 2). E-cadherin/E-cadherin interactions in the cell–cell adhesion region trigger Cdc42-GTP activation [87] and the phosphorylation of aPKC, which in turn phosphorylates LGL. Phosphorylated LGL dissociates from PAR6/aPKC dimer and distributes to the lateral membrane, where it could interact with DLG and SCRIB [85]. aPKC is then able to interact with and phosphorylate PAR3, allowing the formation of the active PAR complex at the apical junctions [69], [159]. A direct connection therefore exists between the activity of the basolateral complex containing LGL and the active apical PAR complex. aPKC is required for the stable localization of PAR3, and PAR3 phosphorylated at S827 residue accumulates at tight junctions [84], [160]. Once it has been phosphorylated at the S827 residue, PAR3 therefore dissociates from aPKC [84]. The existence of this mechanism is supported by the fact that in Drosophila Par1 phosphorylates Par3 inducing the fixation/binding of the adaptor 14.3.3 protein, preventing the association between Par3 and DmPar6/DaPkc dimer [161]. The kinase domain of aPKC is then released and is free to phosphorylate other proteins. aPKC is able to bind directly to the CRB cytoplasmic tail that contains two threonine residues (T6 and T9) in an evolutionarily conserved region, which are potential targets for aPKC phosphorylation. Interestingly, Dpatj is able to modulate the phosphorylation of Crb by aPkc that is required for the proper localization of aPkc and Dpatj to the apical membrane and that of Scrib to the basolateral domain [162]. CRB3 binds to PAR6 directly [53] or via PALS1 [62], to promote the differentiation of the premature junctional structure into mature epithelial structures. In addition, overexpression of both PAR6 and CRB3 in MDCK cells delays the assembly of tight junctions [53], [61], which strongly suggests that PAR6 and CRB3 are involved in the same pathway, leading to the formation of tight junctions. The existence of a relationship between CRB complex and PAR complex has been further supported by the finding that localization of aPKCζ to tight junctions was inhibited in PALS1 siRNA expressing epithelial cells [109], probably via the scaffold protein PAR6 that binds to aPKC and PALS1. Interestingly, PAR3A was not mislocalized in the PALS1 knock down cells, which suggests that PAR3A is not recruited at the apical domain epithelial cells via PAR6, but that it is rather anchored to junctional complexes by JAM proteins present just below the CRB complex [71].