Biochemical analysis of TssK, a core component of the bacterial Type VI secretion system, reveals distinct oligomeric states of TssK and identifies a TssK-TssFG sub-complex

Gram-negative bacteria use the Type VI secretion system (T6SS) to inject toxic proteins into rival bacteria or eukaryotic cells. However the mechanism of the T6SS is incompletely understood. Here we studied a conserved component of the T6SS, TssK, using the anti-bacterial T6SS of Serratia marcescens as a model system. TssK was confirmed to be essential for effector secretion by the T6SS. The native protein, whilst not an integral membrane protein, appeared to localise to the inner membrane, consistent with its presence within a membrane-anchored assembly. Recombinant TssK purified from S. marcescens was found to exist in several stable oligomeric forms, namely trimer, hexamer and higher-order species. Native-level purification of TssK identified TssF and TssG as interacting proteins. TssF and TssG, conserved T6SS components of unknown function, were required for T6SS activity but not for correct localisation of TssK. A complex containing TssK, TssF and TssG was subsequently purified in vitro, confirming these three proteins form a new sub-complex within the T6SS. Our findings provide new insight into the T6SS assembly, allowing us to propose a model whereby TssK recruits TssFG into the membrane-associated T6SS complex and different oligomeric states of TssK may contribute to the dynamic mechanism of the system. © 2014 The Authors Journal compilation © 2014 Biochemical Society