A Model for the Homotypic Interaction between Na+,K+-ATPase ß1 Subunits Reveals the Role of Extracellular Residues 221-229 in Its Ig-Like Domain.

ABSTRACT

The Na+, K+-ATPase transports Na+ and K+ across the membrane of all animal cells. In addition to its ion transporting function, the Na+, K+-ATPase acts as a homotypic epithelial cell adhesion molecule via its ß1 subunit. The extracellular region of the Na+, K+-ATPase ß1 subunit includes a single globular immunoglobulin-like domain. We performed Molecular Dynamics simulations of the ectodomain of the ß1 subunit and a refined protein-protein docking prediction. Our results show that the ß1 subunit Ig-like domain maintains an independent structure and dimerizes in an antiparallel fashion. Analysis of the putative interface identified segment Lys221-Tyr229. We generated triple mutations on YFP-ß1 subunit fusion proteins to assess the contribution of these residues. CHO fibroblasts transfected with mutant ß1 subunits showed a significantly decreased cell-cell adhesion. Association of ß1 subunits in vitro was also reduced, as determined by pull-down assays. Altogether, we conclude that two Na+, K+-ATPase molecules recognize each other by a large interface spanning residues 221-229 and 198-207 on their ß1 subunits.