The Maguk protein, Pals1, functions as an adapter, linking mammalian homologues of Crumbs and Discs Lost

Abstract

Membrane-associated guanylate kinase (Maguk) proteins are scaffold proteins that contain PSD-95–Discs Large–zona occludens-1 (PDZ), Src homology 3, and guanylate kinase domains. A subset of Maguk proteins, such as mLin-2 and protein associated with Lin-7 (Pals)1, also contain two L27 domains: an L27C domain that binds mLin-7 and an L27N domain of unknown function. Here, we demonstrate that the L27N domain targets Pals1 to tight junctions by binding to a PDZ domain protein, Pals1-associated tight junction (PATJ) protein, via a unique Maguk recruitment domain. PATJ is a homologue of Drosophila Discs Lost, a protein that is crucial for epithelial polarity and that exists in a complex with the apical polarity determinant, Crumbs. PATJ and a human Crumbs homologue, CRB1, colocalize with Pals1 to tight junctions, and CRB1 interacts with PATJ albeit indirectly via binding the Pals1 PDZ domain. In agreement, we find that a Drosophila homologue of Pals1 participates in identical interactions with Drosophila Crumbs and Discs Lost. This Drosophila Pals1 homologue has been demonstrated recently to represent Stardust, a crucial polarity gene in Drosophila. Thus, our data identifies a new multiprotein complex that appears to be evolutionarily conserved and likely plays an important role in protein targeting and cell polarity.

Nok plays an essential role in maintaining the integrity of the outer nuclear layer in the zebrafish retina

“…In mammalian MDCK cell line, Crumbs interacts with the PDZ domain of Nok's mammalian homolog Pals1 via the COOH terminus of Crumbs (Roh et al, 2002). The physical interaction between Pals1 and Crb has also been demonstrated in mouse retinal lysate (van de Pavert et al, 2004).…”

“…Besides a PDZ domain, six additional protein-protein interaction domains have been identified in Nok and some of their targets have been identified in mammalian epithelial cell culture systems. Among these domains, the Nterminal conserved domain interacts with Par-6 (Hurd et al, 2003), L27N interacts with DLT (Roh et al, 2002), L27C interact with Lin7 (Kamberov et al, 2000). These proteins also likely interact with Nok in photoreceptors and assist in targeting Crumbs to the inner segments.…”

The transcription factor snail represses Crumbs3 expression and disrupts apico-basal polarity complexes

“…The z-stack image was obtained using an Olympus FluoView 500 laser-scanning confocal microscope, and images were collected in 0.3 mm z-steps ( Â 600). Antibodies used: polyclonal PKCz (Millipore, Billerica, MA, USA); polyclonal Crumbs3, PALS1, PATJ and Par3 were previously described (Roh et al, 2002); monoclonal ZO-1, polyclonal E-cadherin, fluorochrome-conjugated secondary antibodies (Invitrogen, Carlsbad, CA, USA). Two control and four Snail-FLAG clonal cell lines were cultured, harvested in denaturing buffer and 20 mg protein samples resolved by SDS-polyacrylamide gel electrophoresis (PAGE) and immunoblotted for the Par and Crumbs complexes as described (Straight et al, 2006).…”

The tetrameric L27 domain complex as an organization platform for supramolecular assemblies

“…The C-terminal L27 domain of mLin-2 specifically binds to the L27 domain of 15). The discovery of L27 domains as specific protein-protein interaction modules capable of forming heteromeric complexes suggests that L27 domains can integrate multiple scaffold proteins into supramolecular assemblies 1,3,13,15,16 . However, the molecular basis of the L27 domain-mediated protein assembly formation remains unclear.…”