Cysteine-scanning mutagenesis and thiol modification of the Rickettsia prowazekii ATP/ADP translocase: evidence that TM VIII faces an aqueous channel

The contribution of transmembrane region VIII of the Rickettsia prowazekii ATP/ADP translocase to the structure of the water-filled channel through which ATP is transported was evaluated from the accessibility of three hydrophilic, thiol reactive, methanethiosulfonate reagents to a library of 21 single-cysteine substitution mutants expressed in Escherichia coli. A negatively charged reagent (MTSES) and two positively charged reagents (MTSET and MTSEA) were used. Mutants Q323C and G327C did not tolerate cysteine substitution and were almost completely deficient in ATP transport. The remaining mutants exhibited 25-226% of the cysteine-less parent's transport activity. Five patterns of inhibition of ATP transport by the MTS reagents were observed. (i) ATP transport was not inhibited by any of the three MTS reagents in mutants Q321C, F324C, A332C, and L335C and only marginally in F333C. (ii) Transport activity of mutants F322C, Q326C, and A330C was markedly inhibited by all three reagents. (iii) ATP transport was inhibited by MTSEA in only the largest group of mutants (M334C, I336C, G337C, S338C, N339C, I340C, and I341C). (iv) Transport activity was inhibited by MTSET and MTSEA, whereas high concentrations of MTSES were required to inhibit mutants W328C, V329C, and I331C. However, mutant W328C could be inhibited by MTSES in the presence of sub-K(m) concentrations of the substrate. (v) ATP transport by mutant Y325C was unaffected by MTSEA, but inhibited approximately 50% by MTSET and MTSES. Transport of ATP protected mutants (F322C, W328C, V329C, A330C, and I331C) from MTS inhibition. Mutants in the half of TM VIII that is closest to the cytoplasm were not inhibited well by MTSES or MTSET in either whole cells or inside-out vesicles. The results indicate that TM VIII makes a major contribution to the structure of the aqueous translocation pathway, that the accessibility to impermeant thiol reagents is influenced (blocked or stimulated) by substrate, and that there is great variation in accessibility to MTS reagents along the length of TM VIII.     

Cysteine-scanning mutagenesis and thiol modification of the Rickettsia prowazekii ATP/ADP translocase: characterization of TMs IV-VII and IX-XII and their accessibility to the aqueous translocation pathway

We have determined the accessibility of the Rickettsia prowazekii ATP/ADP translocase transmembrane domains (TMs) IV-VII and IX-XII to the putative, water-filled ATP translocation pathway. A library of 177 independent mutants, each with a single cysteine substitution, was expressed in Escherichia coli, and those with substantial ATP transport activity were assayed for inhibition by thiol-reactive, methanethiosulfonate (MTS) reagents. The MTS reagents used were MTSES (negatively charged), MTSET (positively charged), and MTSEA (amphipathic). Inhibition of ATP transport by a charged MTS reagent indicates the exposure of a TM to the water-filled ATP translocation pathway. The eight TMs characterized in this study had 32 mutants with no assayable transport activity, indicating that cysteine substitution at these positions is not tolerated. ATP transport proficient mutants in TMs IV, V, VII, X, and XI were inhibited by charged MTS reagents, indicating that these TMs are exposed to the aqueous ATP translocation pathway, which is a pattern similar to those of TMs I, II (Alexeyev, M. F. (2004) Biochemistry 43, 6995-7002), and VIII (Winkler, H. H. (2003) Biochemistry 42, 12562-12569). Conversely, ATP-transport-proficient mutants in TMs VI, IX, and XII were not inhibited by charged MTS reagents, indicating that these TMs are sequestered from the aqueous environment, which is a pattern similar to that of TM III (Alexeyev, M. F. (2004) Biochemistry 43, 6995-7002). Preexposure of several MTS-sensitive mutants in TMs V, VII, X, and XI to ATP concentrations 10 times the K(m) resulted in protection from MTS-mediated inhibition; thus, confirming exposure of these TMs to the aqueous ATP translocation pathway, a pattern of protection similar to that observed for TMs I, II, and VIII.     

Voltage sensors in domains III and IV, but not I and II, are immobilized by Na+ channel fast inactivation

Using site-directed fluorescent labeling, we examined conformational changes in the S4 segment of each domain of the human skeletal muscle sodium channel (hSkM1). The fluorescence signals from S4 segments in domains I and II follow activation and are unaffected as fast inactivation settles. In contrast, the fluorescence signals from S4 segments in domains III and IV show kinetic components during activation and deactivation that correlate with fast inactivation and charge immobilization. These results indicate that in hSkM1, the S4 segments in domains III and IV are responsible for voltage-sensitive conformational changes linked to fast inactivation and are immobilized by fast inactivation, while the S4 segments in domains I and II are unaffected by fast inactivation.