Biology · Cell Biology

Consider the following statements regarding the selectivity filter of voltage-gated K+ channels: I. The signature sequence TVGYG forms the selectivity filter using backbone carbonyl oxygens to mimic water molecules of K+ hydration shell. II. Na+ passes through K+ channels at about 1/1000th the rate of K+ because Na+ is too large to fit the selectivity filter geometry. III. The selectivity filter has four K+-binding sites (S1–S4), and two K+ ions simultaneously occupy alternate sites (S1,S3 or S2,S4) due to electrostatic repulsion. IV. Tetraethylammonium (TEA) blocks K+ channels by occluding the selectivity filter from the extracellular side. Which of the above statements are correct?