ANALYSIS OF SUBUNIT ASSEMBLY OF THE NA-K-ATPASE

The Na-K-ATPase, or sodium pump, is comprised of two subunits, alpha a nd beta. Each subunit spans the lipid bilayer of the cell membrane. Th is review summarizes our efforts to determine how the two subunits int eract to form the functional ion transporter. Our major approach has b een to observe the potential for subunit assembly when one or both sub units are truncated or present as chimeras that retain only a limited region of the Na-K-ATPase. DNAs encoding these altered subunit forms o f the avian Na-K-ATPase are expressed in mammalian cells. Monoclonal a ntibodies specific for the avian beta-subunit are then used to purify newly synthesized avian beta-subunits, and the presence of accompanyin g alpha-subunits indicates that subunit assembly has occurred. The ect odomain of the beta-subunit (approximately residues 62-304) is suffici ent for assembly with the alpha-subunit, and a COOH-terminal truncatio n of the beta-subunit that lacks aminoacyl residues beyond 162 will as semble inefficiently. A maximum of 26 aminoacyl residues of the a-subu nit are necessary for robust assembly with the P-subunit, when this se quence replaces the COOH-terminal half of the loop between membrane sp ans 7 and 8 in the SERCA1 Ca-ATPase. This region of the Ca-ATPase face s the lumen of the endoplasmic reticulum. These findings encourage stu dy of other related questions, including whether there is preferential assembly of certain subunit isoforms and how various P-type ATPases a re targeted to their appropriate subcellular compartments.