The type II secretion system utilizes GspN for transport of lipoproteins to the Acinetobacter baumannii cell surface

Gram-negative bacteria employ the type II secretion system (T2SS) to not only secrete an array of soluble effectors, such as toxins, to the extracellular space, but also to facilitate the surface localization of enzymes and adhesins that are beneficial to life in different environments. For example, the pullulan-degrading enzyme pullulanase (PulA) from Klebsiella pneumoniae and the recently discovered adhesin InvL from Acinetobacter baumannii are initially expressed with a lipobox containing a signal peptide, resulting in their N-terminal acylation and subsequent surface anchoring after T2SS-mediated export. While outer membrane translocation of both soluble and surface-associated T2SS effectors depends on the T2SS secretin GspD, it is unclear how lipoproteins are accommodated by the T2SS during transport to the cell surface. Here, we identify a role for GspN in the outer membrane translocation of InvL in the opportunistic pathogen A. baumannii. Additional putative lipoproteins are found to have a similar GspN dependence for outer membrane translocation, while soluble proteins are extracellularly secreted in the absence of GspN. We demonstrate that a specific sorting motif C-terminal to the lipobox is required for GspN-dependent surface localization. Based on structural predictions, GspN is homologous to members of the AsmA-like protein family that includes both eukaryotic and prokaryotic proteins containing concave β-taco folds. Although this protein family has been implicated in phospholipid transport, here we show a role for a β-taco fold protein in the transport of lipoproteins. We also confirm that the GspN homolog PulN is required for PulA surface localization in K. pneumoniae.

The type II secretion system (T2SS) is considered a virulence factor of Gram-negative pathogens, such as Acinetobacter baumannii. Despite a described function for the majority of the core components of the T2SS, the role of GspN has been unclear, and it was previously reported that GspN is dispensable for protein secretion by this system. Here, we characterize the selective transport of a subset of proteins by the T2SS and show that GspN is required for their outer membrane translocation and surface localization.