Deletion of a previously uncharacterized lipoprotein lirL confers resistance to an inhibitor of type II signal peptidase in <i>Acinetobacter baumannii</i>.

Abstract

<i>Acinetobacter baumannii</i> is a clinically important, predominantly health care-associated gram-negative bacterium with high rates of emerging resistance worldwide. Given the urgent need for novel antibacterial therapies against <i>A. baumannii</i>, we focused on inhibiting lipoprotein biosynthesis, a pathway that is essential for envelope biogenesis in gram-negative bacteria. The natural product globomycin, which inhibits the essential type II signal peptidase prolipoprotein signal peptidase (LspA), is ineffective against wild-type <i>A. baumannii</i> clinical isolates due to its poor penetration through the outer membrane. Here, we describe a globomycin analog, G5132, that is more potent against wild-type and clinical <i>A. baumannii</i> isolates. Mutations leading to G5132 resistance in <i>A. baumannii</i> map to the signal peptide of a single hypothetical gene, which we confirm encodes an alanine-rich lipoprotein and have renamed <i>lirL</i> (prolipoprotein signal peptidase inhibitor resistance lipoprotein). LirL is a highly abundant lipoprotein primarily localized to the inner membrane. Deletion of <i>lirL</i> leads to G5132 resistance, inefficient cell division, increased sensitivity to serum, and attenuated virulence. Signal peptide mutations that confer resistance to G5132 lead to the accumulation of diacylglyceryl-modified LirL prolipoprotein in untreated cells without significant loss in cell viability, suggesting that these mutations overcome a block in lipoprotein biosynthetic flux by decreasing LirL prolipoprotein substrate sensitivity to processing by LspA. This study characterizes a lipoprotein that plays a critical role in resistance to LspA inhibitors and validates lipoprotein biosynthesis as a antibacterial target in <i>A. baumannii</i>.