The beta subunit of Na,K-ATPase is required for stabilization and maturation of the catalytic alpha subunits and is also involved in cell adhesion and establishing epithelial cell polarity. However, the mechanism of cell adhesion effects and protein partners of beta are unknown. We have applied fold recognition methods to predict that a C-terminal domain of the beta subunits of Na,K-ATPase and H,K-ATPase has an immunoglobulin-like fold, which resembles cell adhesion molecules. Comparison of the predicted C-terminal domain with a recently published structure of shark rectal gland Na,K-ATPase at 2.4 A in which alpha, beta, and FXYD subunits were resolved confirms that the beta subunit ectodomain contains an immunoglobulin-like structure. Expression in Escherichia coli of a sequence corresponding to the C-terminal domain, followed by its purification, refolding, and circular dichroism analysis, shows that the domain is independently stable with prominent beta sheet secondary structure, as predicted. Proteolytic digestion of the purified detergent-soluble recombinant Na,K-ATPase (alpha1beta1) is also indicative of a stable C-terminal domain of beta in the native complex. The major conclusion of this work is consistent with prior evidence for a role of the beta subunit in cell-cell adhesion, and it attributes that function largely to the C-terminal lobe of the beta ectodomain. In the light of these findings, we discuss its role in cell adhesion and recognition of the beta subunits of Na,K-ATPase, including potential protein partners.