Reported LQT1-associated S3 mutants traffic to the plasma membrane in ltk− cells. (A) Western blot of WT and mutant KCNQ1+KCNE1 proteins expressed in ltk− cells, untreated and treated with proteinase K (PK) to distinguish surface-expressed channel from internal. After normalization to actin for control of loading, the percent surface protein was determined from the density of bands before and after proteinase K treatment (n = 3) and was found to be: WT, 73%; D202H, 91%; D202N, 80%; I204F, 80%; I204M, 90%; V205M, 75%; S209F, 82%; V215M, 75%. (B) Model of KCNQ1 tetrameric channels showing the locations of S3 mutations in space fill from the side and the extracellular face, with each subunit a different color.