Fig. 4.

Cooperative translational integration of Shaker S3–S4 into the membrane. (A and B) All six conserved positively charged residues (R or K) in S4 in the same S4 model protein as in Fig. 2A were replaced by Q (R/K-lessS4) and the SA-I, SA-II, translocation-reinitiating, and stop-transfer functions of the R/K-lessS4 determined in the same manner as in Fig. 2. The data on assessments for SA-I, SA-II, and translocation-reinitiating functions are also shown in SI Fig. 10. (C and D) Membrane integration of S3–S4 in the S1–S4 construct in which S4 was replaced by R/K-lessS4 to remove the electrostatic interaction between S2/S3 and S4. The efficiency of integration (%) is shown in G. (E and F) Assessment of the contribution of positive charges in S4 to the membrane integration of S3–S4. Increasing numbers of R or K residues were added sequentially to S4 of the S1–R/K-lessS4 construct, starting at the N terminus. The efficiencies of integration are shown in G.