Triplin: Mechanistic Basis for Voltage Gating

Abstract

The outer membrane of Gram-negative bacteria contains a variety of pore-forming structures collectively referred to as porins. Some of these are voltage dependent, but weakly so, closing at high voltages. Triplin, a novel bacterial pore-former, is a three-pore structure, highly voltage dependent, with a complex gating process. The three pores close sequentially: pore 1 at positive potentials, 2 at negative and 3 at positive. A positive domain containing 14 positive charges (the voltage sensor) translocates through the membrane during the closing process, and the translocation is proposed to take place by the domain entering the pore and thus blocking it, resulting in the closed conformation. This mechanism of pore closure is supported by kinetic measurements that show that in the closing process the voltage sensor travels through most of the transmembrane voltage before reaching the energy barrier. Voltage-dependent blockage of the pores by polyarginine, but not by a 500-fold higher concentrations of polylysine, is consistent with the model of pore closure, with the sensor consisting mainly of arginine residues, and with the presence, in each pore, of a complementary surface that serves as a binding site for the sensor.

Beta-Barrel Channel Response to High Electric Fields: Functional Gating or Reversible Denaturation?

“…Could a voltage of sufficient magnitude to induce channel closing even be attained across the outer membrane? For some E. coli outer membrane porins, such as OmpC [87], the "crucial voltage", a vague term intentionally sidestepped in the present article, was reported to be close to 200 mV, whereas other porins, e.g., the recently discovered putative β-barrel channel Triplin [88] and Vibrio cholerae OmpT exhibit voltage-induced closing at considerably lower transmembrane voltages [89]. At the same time, an increase in solution acidity [90] and the presence of polyamines, such as spermine, spermidine, and cadaverine, are cofactors known to increase the voltage sensitivity of OmpF [16,22,91].…”

“…There are additional reports showing that β-barrel channels can undergo gating under the application of low, physiologically relevant voltages [88,89], thus suggesting gating functionality, or exhibiting a prominent asymmetry toward the applied voltage sign [72,[101][102][103][104]. Over the past 25 years, researchers from our laboratories have collectively investigated eleven distinct β-barrel channel-forming proteins, encompassing five outer membrane bacterial porins, five bacterial exotoxins, and mitochondrial VDAC.…”