Abstract
Contractile injection systems (CISs) [type VI secretion system (T6SS), phage tails, and tailocins] use a contractile sheath-rigid tube machinery to breach cell walls and lipid membranes. The structures of the pre- and postcontraction states of several CISs are known, but the mechanism of contraction remains poorly understood. Combining structural information of the end states of the 12-megadalton R-type pyocin sheath-tube complex with thermodynamic and force spectroscopy analyses and an original modeling procedure, we describe the mechanism of pyocin contraction. We show that this nanomachine has an activation energy of 160 kilocalories/mole (kcal/mol), and it releases 2160 kcal/mol of heat and develops a force greater than 500 piconewtons. Our combined approach provides a quantitative and experimental description of the membrane penetration process by a CIS.
Highlights
Contractile injection systems (CISs), which include the bacterial type VI secretion system (T6SS), bacteriophage tails, R-type pyocins, and other tailocins, function to penetrate bacterial and eukaryotic membranes [1,2,3,4]
The model describes the structure of the highest energy state, which enables the manipulation of the activation energy by targeted mutagenesis
The Domain Motion in Atomic Detail (DMAD) method is innovative in that it can provide physically realistic transition intermediates for systems that are too large for a conventional molecular dynamics (MD) study [34]
Summary
Contractile injection systems (CISs), which include the bacterial type VI secretion system (T6SS), bacteriophage tails, R-type pyocins, and other tailocins, function to penetrate bacterial and eukaryotic membranes [1,2,3,4]. The universally conserved part of CISs consists of an external contractile sheath, an internal rigid tube, and a baseplate (Fig. 1A). The baseplate-distal end of the tube and the sheath are fixed to each other with a capping protein (Fig. 1A). Contraction of the sheath results in the motion of the tube toward and through the target cell membrane (Fig. 1A) [7]. This process is aided by a spike-shaped protein located at the baseplate- proximal end of the tube (Fig. 1A) [9, 10]. The membrane-attacking tip of the spike protein is stabilized by an iron or zinc atom [9, 10]
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