Modulation of the Shaker K+ channel gating kinetics by the S3-S4 linker

In Shaker K+ channels depolarization displaces outwardly the positively cha rged residues of the S4 segment. The amount of this displacement is unknown , but large movements of the S4 segment should be con strained by the lengt h and flexibility of the S3-S4 Linker. To investigate the role of the S3-S4 linker in the ShakerH4 Delta(6-46) (Shaker Delta) K+ channel activation, w e constructed S3-S4 linker deletion mutants. Using macro-patches of Xenopus oocytes, we tested three constructs: a deletion mutant with no linker (0 a a linker), a mutant containing a linker 5 amino acids in length, and a 10 a mino acid linker mutant. Each of the three mutants tested yielded robust K currents. The half-activation voltage was shifted to the right along the v oltage axis, and the shift was +45 mV in the case of the 0 aa linker channe l. In the 0 aa linker, mutant deactivation kinetics were sixfold slower tha n in Shaker Delta. The apparent number of gating charges was 12.6 +/- 0.6 e (o) in Shaker Delta, 12.7 +/- 0.5 in 10 aa linker, and 12.3 +/- 0.9 in 5 aa linker channels, but it was only 5.6 +/- 0.3 e(o) in the 0 aa linker mutan t channel. The maximum probability of opening (P-o(max)) as measured using noise analysis was not altered by the linker deletions. Activation kinetics were most affected by linker deletions; at 0 mV, the 5 and 0 aa linker cha nnels' activation time constants were 89x and 45x slower than that of the S haker Delta K+ channel, respectively. The initial lag of ionic currents whe n the prepulse was varied from -130 to -60 mV was 0.5, 14, and 2 ms for the 10, 5, and 0 aa linker mu rant channels, respectively These results sugges t that: (a) the 34 segment moves only a short distance during activation si nce an S3-S4 linker consisting of only 5 amino acid residues allows for the total charge displacement to occur; and (b) die length of tl-le S3-S4 link er plays an important role in setting Shaker Delta channel activation and d eactivation kinetics.