An ATP-dependent L-carnitine transporter in Listeria monocytogenes Scott A is involved in osmoprotection.

Listeria monocytogenes is a gram-positive, psychotrophic, food-borne pathogen which is able to grow in osmotically stressful environments. Carnitine (beta-hydroxy-L-tau-N-trimethyl aminobutyrate) can contribute significantly to growth of L. monocytogenes at high osmolarity (R. R. Beumer, M. C. te Giffel, L. J. Cox, F. M. Rombouts, and T. Abee, Appl. Environ. Microbiol. 60:1359-1363, 1994). Transport of L-[N-methyl-14C]carnitine in L. monocytogenes was shown to be energy dependent. Analysis of cell extracts revealed that L-carnitine was not further metabolized, which supplies evidence for its role as an osmoprotectant in L. monocytogenes. Uptake of L-carnitine proceeds in the absence of a proton motive force and is strongly inhibited in the presence of the phosphate analogs vanadate and arsenate. The L-carnitine permease is therefore most likely driven by ATP. Kinetic analysis of L-carnitine transport in glucose-energized cells revealed the presence of a high-affinity uptake system with a Km of 10 microM and a maximum rate of transport (Vmax) of 48 nmol min-1 mg of protein-1. L-[14C]carnitine transport in L. monocytogenes is significantly inhibited by a 10-fold excess of unlabelled L-carnitine, acetylcarnitine, and tau-butyrobetaine, whereas L-proline and betaine display, even at a 100-fold excess, only a weak inhibitory effect. In conclusion, an ATP-dependent L-carnitine transport system in L. monocytogenes is described, and its possible roles in cold adaptation and intracellular growth in mammalian cells are discussed.