Science, published online 17 November 2011, doi:10.1126/science.1212868

A type IV secretion system delivers Legionella pneumophila effectors into human host cells to promote bacterial proliferation within Legionella-containing vacuoles (LCVs). AnkB, an F-box protein effector that assembles polyubiquitinated (polyUb) proteins on the LCV, is required for bacterial proliferation. Price et al. demonstrate that disruption of ubiquitination via Lys48—the linkage associated with proteasomal degradation—decreased recruitment of polyUb proteins to LCVs and bacterial proliferation. Inhibition of the proteasome with small molecules decreased bacterial replication in host cells, and this phenotype could be rescued by amino acid supplementation, observations suggesting that degradation of polyUb proteins provides amino acids to support bacterial growth. Similarly, bacterial proliferation defects in host cells treated with small-molecule inhibitors of aminopeptidases—enzymes that degrade peptides—or in those overexpressing ubiquitin with a K48R substitution were also rescued by amino acid supplementation. MS revealed an increase in free amino acids in cells infected with Legionella (compared to uninfected cells) that was dependent upon AnkB expression. Supplementation with cysteine or serine alone was sufficient to rescue proliferation, suggesting that the bacteria may use amino acids as an energy source. Consistent with this idea, supplementation with citrate or pyruvate, intermediates in the tricarboxylic acid cycle, also rescued proliferation. Taken together, these data indicate that for replicating Legionella, amino acids produced by promoting the proteasomal degradation of host proteins are a source of energy, carbon or both.