Topogenesis of two transmembrane type K+ channels, Kir 2.1 and KcsA.

Potassium channels, which control the passage of K+ across cell membranes, have two transmembrane segments, M1 and M2, separated by a hydrophobic P region containing a highly conserved signature sequence. Here we analyzed the membrane topogenesis characteristics of the M1, M2, and P regions in two animal and bacterial two-transmembrane segment-type K+ channels, Kir 2.1 and KcsA, using an in vitro translation and translocation system. In contrast to the equivalent transmembrane segment, S5, in the voltage-dependent K+ channel, KAT1, the M1 segment in KcsA, was found to have a strong type II signal-anchor function, which favors the Ncyt/Cexo topology. The N-terminal cytoplasmic region was required for efficient, correctly orientated integration of M1 in Kir 2.1. Analysis of N-terminal modification by in vitro metabolic labeling showed that the N terminus in Kir 2.1 was acetylated. The hydrophobic P region showed no topogenic function, allowing it to form a loop, but not a transmembrane structure in the membrane; this region was transiently exposed in the endoplasmic reticulum lumen during the membrane integration process. M2 was found to possess a stop-transfer function and a type I signal-anchor function, enabling it to span the membrane. The C-terminal cytoplasmic region in KcsA was found to affect the efficiency with which the M2 achieved their final structure. Comparative topogenesis studies of Kir 2.1 and KcsA allowed quantification of the relative contributions of each segment and the cytoplasmic regions to the membrane topology of these two proteins. The membrane topogenesis of the pore-forming structure is discussed using results for Kir 2.1, KcsA, and KAT1.