9.C.20.  The Outer Membrane Porin, Triplin (Triplin) Family 

The outer membranes of Gram-negative bacteria contain 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 (Colombini et al. 2023). 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 (Colombini et al. 2023). Dr. Colombini, the corresponding author, says the protein has not yet been isolated, so a sequence is not available. [If so, how can he know that 14 positive residues comprise the voltage sensor?]



Colombini, M., P. Liu, and C. Dee. (2023). Triplin: Mechanistic Basis for Voltage Gating. Int J Mol Sci 24:.

Kojima, S., M. Iwamoto, S. Oiki, S. Tochigi, and H. Takahashi. (2018). Thylakoid membranes contain a non-selective channel permeable to small organic molecules. J. Biol. Chem. 293: 7777-7785.