Cell type-specific recruitment of Drosophila Lin-7 to distinct MAGUK-based protein complexes defines novel roles for Sdt and Dlg-S97.

André Bachmann,Marco Timmer,Ulrich Thomas,Jimena Sierralta,Grazia Pietrini,Elisabeth Knust,Eckart D Gundelfinger

doi:10.1242/jcs.01029

Abstract

Stardust (Sdt) and Discs-Large (Dlg) are membrane-associated guanylate kinases (MAGUKs) involved in the organization of supramolecular protein complexes at distinct epithelial membrane compartments in Drosophila. Loss of either Sdt or Dlg affects epithelial development with severe effects on apico-basal polarity. Moreover, Dlg is required for the structural and functional integrity of synaptic junctions. Recent biochemical and cell culture studies have revealed that various mammalian MAGUKs can interact with mLin-7/Veli/MALS, a small PDZ-domain protein. To substantiate these findings for their in vivo significance with regard to Sdt- and Dlg-based protein complexes, we analyzed the subcellular distribution of Drosophila Lin-7 (DLin-7) and performed genetic and biochemical assays to characterize its interaction with either of the two MAGUKs. In epithelia, Sdt mediates the recruitment of DLin-7 to the subapical region, while at larval neuromuscular junctions, a particular isoform of Dlg, Dlg-S97, is required for postsynaptic localization of DLin-7. Ectopic expression of Dlg-S97 in epithelia, however, was not sufficient to induce a redistribution of DLin-7. These results imply that the recruitment of DLin-7 to MAGUK-based protein complexes is defined by cell-type specific mechanisms and that DLin-7 acts downstream of Sdt in epithelia and downstream of Dlg at synapses.

Highlights

Many cell types exhibit an asymmetric subdivison of the plasma membrane and the underlying cytocortex into discrete domains with specialized structural and functional properties
Recent biochemical and cell culture studies have revealed that various mammalian membraneassociated guanylate kinases (MAGUKs) can interact with mLin-7/Veli/MALS, a small PDZ-domain protein
To substantiate these findings for their in vivo significance with regard to Sdt- and Dlg-based protein complexes, we analyzed the subcellular distribution of Drosophila Lin-7 (DLin-7) and performed genetic and biochemical assays to characterize its interaction with either of the two MAGUKs

Summary

Introduction

Many cell types exhibit an asymmetric subdivison of the plasma membrane and the underlying cytocortex into discrete domains with specialized structural and functional properties This is mediated by the differential localization of distinct, membrane-anchored protein complexes including those forming cellular junctions. The precise spatial arrangement of such complexes relies on the efficient sorting and selective retention of their components Studies addressing these requirements have revealed that complexspecific combinations of submembraneous proteins provide a scaffold of adapter modules, primarily PDZ-type domains, to which transmembrane proteins and cytoplasmic components can bind concomitantly (Sheng and Sala, 2001; Harris and Lim, 2001). This concept is well-established for membrane-associated guanylate kinases (MAGUKs), a superfamily of evolutionary conserved scaffolding proteins

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