Figure 4.

M7 as well as the extracytoplasmic loop between M7 and M8 of the Na,K-ATPase α subunit contain putative degradation signals. Oocytes were injected with cRNA coding for truncated α-proteins in the absence (lanes 1–14) or presence (lanes 15 and 16) of cRNA coding for β subunits, treated as described in Figure 1 and immunoprecipitated with an α antibody under denaturing conditions (lanes 1–14) or nondenaturing conditions (lanes 15 and 16). Complete membrane insertion of M7 does not prevent degradation of M1–7 α-proteins, and transposition of the extracytoplasmic loop between M7 and M8 (el) mediates degradation of intrinsically stable, truncated M1–4 α-proteins. Also, β subunits can associate with and stabilize M1–3/el/4 α-proteins. Indicated are the positions of α-proteins. The β subunit coimmunoprecipitated with M1–3/el/4 α-proteins is indicated by an asterisk. Immunoprecipitated samples shown in lanes 15 and 16 were treated with Endo H to permit separation of the α and β bands. One of two or three representative experiments is shown. The putative membrane topology of truncated α-proteins expressed in the absence or presence of β subunits is shown.