The protein-conducting channel, or translocon, is an evolutionarily conserved complex that allows nascent proteins to cross a cellular membrane or integrate into it. The crystal structure of an archaeal translocon, the SecY complex, revealed that two elements contribute to sealing the channel: a small " plug " domain blocking the periplasmic region of the channel, and a pore ring composed of six hydrophobic residues acting as a constriction point at the channel ' s center. To determine the independent functions of these two elements, we have performed molecular dynamics simulations of the native channel as well as of two recently structurally resolved mutants in which portions of their plugs were deleted. We fi nd that in the mutants, the instability in the plug region leads to a concomitant increase in fl exibility of the pore ring. The instability is quantifi ed by the rate of water permeation in each system as well as by the force required for oligopeptide translocation. Through a novel simulation in which the interactions between the plug and water were independently controlled, we fi nd that the role of the plug in stabilizing the pore ring is signifi cantly more important than its role as a purely steric barrier.
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