Cooperativity between trans and cis interactions in cadherin-mediated junction formation

Abstract

Intercellullar junctions formed by cadherins, including desmosomes and adherens junctions, comprise two dimensional arrays of "trans" dimers formed between monomers emanating from opposing cell surfaces. Lateral "cis" interfaces between cadherins from the same cell surface have been proposed to play a role in cadherin clustering. Although the molecular details of cis interactions remain uncertain, they must define an anisotropic arrangement where binding is favorable only in certain orientations. Here we report Monte Carlo simulations performed on a 2D lattice constructed to account for the anisotropy in cadherin cis interactions. A crucial finding is that the "phase transition" between freely diffusing cadherin monomers and dimers and a condensed ordered 2D junction formed by dimers alone is a cooperative process involving both trans and cis interactions. Moreover, cis interactions, despite being too weak to be measured in solution, are critical to the formation of an ordered junction structure. We discuss these results in light of available experimental information on cadherin binding free energies that are transformed from their bulk solution values to interaction energies on a 2D lattice.

Fig. 1.

Trans dimer organization, as derived from the crystal structure of C-cadherin (6).

Fig. 2.

(A) Schematic illustration of two interacting cadherin-decorated membranes. The EC1 domains are labeled by solid ellipses—green for cadherins in the lower membrane and red for those at the top layer. If apposed EC1 domains form a trans dimer they are labeled solid blue. Hollow ellipses label the EC2-EC5 domains. The gray “ghost” molecules illustrate alternative configurations, all within the interaction shell of thickness h. (B) The 2D square lattice, corresponding schematically to a top view on the interaction shell containing all EC1 domains, here depicted as dipoles (monomers). Each lattice site can accommodate no more than one top layer monomer (red dipole), nor more than one bottom layer monomer (green dipole). However, double occupancy by monomers belonging to apposed membranes is a necessary condition for trans dimer formation, provided the angle between the bottom and top monomers is 90° (indicated by a blue solid circle). The cis interactions between monomers and dimers are described in detail in the text.

Fig. 3.

Three simulation snapshots of the anisotropic lattice model. In all cases the overall monomer concentrations are 16% (i.e., ). For clarity we use here a simpler notation, whereby red and green circles represent cadherins from the top and bottom membranes, and the larger, blue, circles denote trans dimers. (A) and Δg⁰(cis) = 0. The monomers and trans-dimers are in chemical equilibrium with each other, randomly dispersed among the lattice sites. (B) Both cis and trans interactions are sizeable: Δg⁰(cis) = 4kT and Δg⁰(trans) = 6kT. The concentration of dimers is large enough and the lateral interaction between them strong enough to drive their condensation into well-defined ordered junctions. (C) Trans binding is weak, Δg⁰(trans) = 2kT, and cis interactions are as large as in B, Δg⁰(cis) = 4kT. The laterally attracting monomers aggregate into linear oligomers which in some cases involve the monomers belonging to trans dimers. The overall concentration of trans dimers is below the threshold concentration necessary for their 2D condensation into dimers.

Fig. 4.

Phase boundaries between the dilute one-phase region (left and above each curve) and the two-phase region, marking the appearance of a condensed junction phase consisting of trans-dimers.

Fig. 5.

A diffusion trap comprising 10 × 10 lattice sites in the center of a 2D lattice of 50 × 50 sites, with periodic boundary conditions. Trans dimer formation can only take place in the central zone—the trap. The distance between membranes in the surrounding region is too large to allow trans dimer formation. In both snapshots shown, Δg⁰(trans) = 6kT and the average monomer density is x⁰ = 0.04. The trap area is 4% of the total area. (A) No lateral attraction between monomers and/or dimers, Δg⁰(cis) = 0. After equilibration the overall concentration in the trap increases to nearly 14%. (B) Here, Δg⁰(cis) = 4kT. Monomers migrate into the trap where they dimerize, enriching the trans dimer population in the trap. The strongly attracting dimers condense into an ordered island attracting additional cadherin molecules into the trap. The higher concentration of cadherins in the trap enahances their dimerization, while the strong cis binding encourages their lateral condensation into a junction.