Abstract
Cadherin extracellular domain 1 (EC1) mediates dimerization of type II cadherins between opposing cell surfaces to facilitate cell-cell adhesion. EC1 forms domain-swapped dimers, with residues Trp2 and Trp4 crucial to anchor the EC1 A-strand in the strand-swapped conformation. In the present work, the specific roles of Trp2 and Trp4 in the dimerization mechanism of Cadherin-11 have been elucidated using NMR spectroscopy of wild-type and designed mutant EC1 domains. The results show that the dominant monomeric state, with the A strand and Trp side chains packed intramolecularly, is in equilibrium with a sparsely populated (1.6%) partially strand-exposed state, in which the Trp2 side chain packing is disrupted and with a sparsely populated (1.6%) fully strand-exposed state, in which the A strand, Trp2 and Trp4 side chain packing are fully disrupted. The exchange kinetics between the major state and the partially strand-exposed state are slow-intermediate (kex = 700 – 734 s-1) and the exchange kinetics between the major state and the fully strand-exposed state are intermediate-fast (kex = 3470 - 3591 s-1,) on the NMR chemical shift time scale. These three conformations, varying in the degree of A-strand exposure, also are coupled to additional conformational states on very fast and very slow timescales. The very fast exchange process arises from interconversion between ordered and random coil conformations of the BC loop in proximity to the Trp2 binding pocket, with relative populations that depend on the extent of A-strand exposure and dimerization status. The very slow exchange processes link the folded partially and fully strand-exposed conformations with partially unfolded conformational states, which have been recognized as crucial intermediates for domain-swapping in proteins other than cadherins. This framework reveals the dimerization mechanism of type II Cadherins as coupled folding and strand-swapping.
Competing Interest Statement
The authors have declared no competing interest.