Abstract
The mammalian MAGI proteins play important roles in the maintenance of adherens and tight junctions. The MAGI family of proteins contains modular domains such as WW and PDZ domains necessary for scaffolding of membrane receptors and intracellular signaling components. Loss of MAGI leads to reduced junction stability while overexpression of MAGI can lead to increased adhesion and stabilization of epithelial morphology. However, how Magi regulates junction assembly in epithelia is largely unknown. We investigated the single Drosophila homologue of Magi to study the in vivo role of Magi in epithelial development. Magi is localized at the adherens junction and forms a complex with the polarity proteins, Par3/Bazooka and aPKC. We generated a Magi null mutant and found that Magi null mutants were viable with no detectable morphological defects even though the Magi protein is highly conserved with vertebrate Magi homologues. However, overexpression of Magi resulted in the displacement of Baz/Par3 and aPKC and lead to an increase in the level of PIP3. Interestingly, we found that Magi and Baz functioned in an antagonistic manner to regulate the localization of the apical polarity complex. Maintaining the balance between the level of Magi and Baz is an important determinant of the levels and localization of apical polarity complex.
Highlights
A common component of junctional and polarity complexes is modular scaffolding proteins that are capable of binding to each other as well as recruiting other proteins to the complex
We examined the localization of Magi with respect to the adherens junction protein E-cadherin (Ecad) and found that Magi colocalizes with Ecad (Fig 1W–1Y)
Signal was concentrated within the apical domain of the columnar epithelia for Magi and Baz:: GFP (Fig 2O–2Q), similar to the control PLA between Baz::GFP and aPKC (Fig 2L–2N), while the background PLA was randomly dispersed in controls (Fig 2R–2T). These results suggest that Magi is in close proximity with the members of the polarity complex at the adherens junction domain in epithelia of embryos and third instar imaginal discs
Summary
A common component of junctional and polarity complexes is modular scaffolding proteins that are capable of binding to each other as well as recruiting other proteins to the complex. Magi proteins are evolutionarily conserved scaffolding proteins and contain multiple domains including a N-terminal catalytically inactive GUK domain, two WW domains and five to six PDZ (PSD95/Dlg/ZO-1) domains [1]. There are three MAGI proteins in vertebrates (MAGI1,2,3) all with multiple splice isoforms. MAGI-1 and MAGI-3 are relatively ubiquitously expressed and localize to a range of junctions including epithelial tight junctions. PLOS ONE | DOI:10.1371/journal.pone.0153259 April 13, 2016
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