Par-3 family proteins in cell polarity & adhesion

Abstract

The Par-3/Baz family of polarity determinants is highly conserved across metazoans and includes C. elegans PAR-3, Drosophila Bazooka (Baz), human Par-3 (PARD3), and human Par-3-like (PARD3B). The C. elegans PAR-3 protein localises to the anterior pole of asymmetrically dividing zygotes with cell division cycle 42 (CDC42), atypical protein kinase C (aPKC), and PAR-6. The same C. elegans 'PAR complex' can also localise in an apical ring in epithelial cells. Drosophila Baz localises to the apical pole of asymmetrically dividing neuroblasts with Cdc42-aPKC-Par6, while in epithelial cells localises both in an apical ring with Cdc42-aPKC-Par6 and with E-cadherin at adherens junctions. These apical and junctional localisations have become separated in human PARD3, which is strictly apical in many epithelia, and human PARD3B, which is strictly junctional in many epithelia. We discuss the molecular basis for this fundamental difference in localisation, as well as the possible functions of Par-3/Baz family proteins as oligomeric clustering agents at the apical domain or at adherens junctions in epithelial stem cells. The evolution of Par-3 family proteins into distinct apical PARD3 and junctional PARD3B orthologs coincides with the emergence of stratified squamous epithelia in vertebrates, where PARD3B, but not PARD3, is strongly expressed in basal layer stem cells - which lack a typical apical domain. We speculate that PARD3B may contribute to clustering of E-cadherin, signalling from adherens junctions via Src family kinases or mitotic spindle orientation by adherens junctions in response to mechanical forces.

Keywords: E-cadherin; cell adhesion; epithelia; neuroblast; polarity; stem cell.

Fig. 1

Role of Par‐3/Baz in cell polarity in neuroblasts and epithelia. (A) The Drosophila Par‐3 homolog Baz localises with the Cdc42‐Par6‐aPKC complex to the apical pole of neural stem cells at mitosis. Clustering of complexes via oligomeric interactions is crucial for formation of a polarised apical plasma membrane domain. (B) Protein–protein interactions upon activation of the Drosophila Cdc42‐Par6‐aPKC complex and its association with Baz. (C) Localisation of the Baz and Cdc42‐aPKC‐Par6 complex in an apical ring just above adherens junctions in epithelial cells (where it acts in parallel with Crumbs (Crb) protein, shown at the right).

Fig. 2

Molecular mechanisms of Par‐3/Baz clustering at the apical domain. (A) The Par‐3/Baz N‐terminal PB1‐like oligomerisation domain may either undergo dynamic and transient homomeric interactions or form a stable helical structure as observed in crystallographic studies (PDB: 3ZEE). Crystal structures of several key protein domains and their interactions have been determined in the Baz‐Par6‐aPKC‐Cdc42 complex (PDB: 5OAK, 2OGP, 1NF3, 2K1Z, 5LI1, 1WMH). (B) Higher order oligomeric clustering of Par‐3/Baz complexes may involve not only the N‐terminal PB1‐like oligomerisation domain, but also interactions between the low complexity ‘coiled‐coil’ domain (which may instead function an intrinsically disordered region) at the C‐terminus of Par‐3/Baz.

Fig. 3

Role of Par‐3/Baz proteins at adherens junctions. (A) Drosophila Baz can separate from the apical PAR complex and localise to adherens junctions when phosphorylated by aPKC. (B) Diagram of interactions between Baz, Par‐6 and aPKC. (C) aPKC phosphorylation of human Par‐3 (PARD3) occurs on S827 within the aPKC interaction site. (D) A loosely bound substrate conformation may promote phosphorylation by aPKC. (E) Comparison of aPKC kinase interaction sites of Par‐3/Baz family members from different species suggests that tight binding promotes stable complex formation with aPKC‐Par6 apically, while loose binding promotes phosphorylation and dissociation from the aPKC‐Par6 complex (such that it colocalises with E‐cadherin). (F) Baz‐GFP can be detected both apically (with aPKC) and laterally (with E‐cad). (G) Human PAR‐3 (PARD3) localises apically, while PAR‐3‐like (PARD3B) localises laterally in monolayer columnar epithelial cells from colorectal cancer biopsies. Data from the www.proteinatlas.org database.

Fig. 4

PARD3 overlaps with CRB3 apically, while PARD3B overlaps with E‐cadherin and Src laterally in monolayered columnar epithelia. Images of CRB3, PARD3, E‐cadherin, PARD3B and SRC from the www.proteinatlas.org database. Four different monolayered columnar epithelial tissues are shown, with PARD3 always apically localised and PARD3B always laterally localised with E‐cadherin and Src. Note that bladder urothelium is a monolayered pseudostratified epithelium.

Fig. 5

PARD3B overlaps with E‐cadherin and Src in basal layer cells of various stratified epithelia. Images of CRB3, PARD3, E‐cadherin, PARD3B and SRC from the www.proteinatlas.org database. Four different stratified epithelial tissues are shown. Note that PARD3 is not expressed in stratified squamous skin, but remains expressed and apical in stratified columnar epithelia. PARD3B and Src are restricted to the basal layer cells of skin, prostate, epididymis and bronchus.

Fig. 6

PARD3B expression in tumours arising from basal layer stem cells and monolayer epithelia. Images of PARD3B from the www.proteinatlas.org database. Four different tumour types are shown. The role of PARD3B in tumour growth and progression remains unknown.