pumping beta subunit (Gcdβ). A proposed structure of the holoenzyme positions the water-filled central channel of the Gcdα dimer coaxial with the ion channel in Gcdβ. The central channel is blocked by arginines, which could allow Na+ passage by conformational movement or by entry through two side channels. The β-subunits possess 9 transmembrane α-helical spanners (TMSs). The protein may dip into the membrane twice between TMSs III and IV. The most conserved regions are segments IIIa, the first membrane loop following TMS III, and TMS VIII. Conserved residues therein, D203 (IIIa), Y229 (IV) and N373, G377, S382 and R389 (VIII), provide Na+ binding sites and the translocation pathway. D203 and S382 may provide two binding sites for the two Na+ ions. D203 is absolutely essential for function and may provide the primary intramembranous Na+-binding site. The beta subunits of these transporters show sufficient sequence similarity to the Na+:H+ antiporters of the CPA2 family (TC #2.A.37) to establish homology (K. Studley and M.H. Saier, Jr., unpublished results). See also Decarboxylase Membrane proteins Transport proteins Transporter Classification Database References Further reading Protein families Membrane proteins Transmembrane proteins Transmembrane transporters Transport proteins Integral membrane proteins