Structural characterization of TssL from Acinetobacter baumannii : a key component of the type VI secretion system

Federico M Ruiz, Juvenal Lopez, C Gastón Ferrara, Elena Santillana, Yanis R Espinosa, Mario F Feldman, Antonio Romero
Journal of Bacteriology 2020 June 22
The type VI secretion system (T6SS) is a complex molecular nanomachine used by Gram-negative bacteria to deliver diverse effectors into adjacent cells. A membrane complex (MC) anchors this transport system to the bacterial cell wall. One of the proteins forming the MC is TssL, a cytoplasmic protein bound to the inner membrane through a single transmembrane helix. Here, we report the structure of the cytoplasmic N-terminal region of TssL from Acinetobacter baumannii , a bacteria encoding in a single locus a secretion system that is a special case among others T6SSs. The protein structure, consisting of two antiparallel alpha-helical bundles connected by a short loop, reveals several interesting particularities when compared with homologues proteins from other organisms. In addition, we demonstrate the structural significance of residues Asp98 and Glu99, which are strongly conserved among T6SS-encoding Gram-negative bacteria. Mutations in these two residues strongly impact protein dynamics, expression and functionality. Our results improve our understanding of the T6SS of A. baumannii , which remains largely understudied, compared with that of other pathogens. IMPORTANCE Several Acinetobacter species carry one functional Type 6 Secretion System (T6SS). The T6SS is encoded in a single locus containing 16 conserved genes, most of which code for proteins essential to T6SS activity. One of these key components is TssL, a cytoplasmic protein bound to the inner membrane. Despite its importance, and its particular characteristics, the structure of T6SS in A. baumannii remains understudied. Here we present structural, in silico and in vivo studies of TssL highlighting the importance of two well-conserved residues and improving our understanding about this secretion system in these bacteria.

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