INHIBITION OF SPIRALIN PROCESSING BY THE LIPOPEPTIDE ANTIBIOTIC GLOBOMYCIN

The cyclic lipopeptide globomycin, a specific inhibitor of signal-pept idase II (Lsp A), proved toxic for the mollicute Spiroplasma melliferu m with a minimal inhibitory concentration (MIG) in the range 6.25-12.5 mu M, about one order of magnitude higher (that is, less efficient) t han bee-venom mellitin. SDS-PAGE analysis of cell proteins followed by immunolabeling (''Western blotting'') and by crossed immunoelectropho resis demonstrated that the cleavage of the prespiralin leader peptide was prevented by globomycin. Cell fractionation experiments showed th at prespiralin was membrane bound and did not accumulate in the cytopl asm or in the culture medium. Furthermore, the use of the potential-se nsitive fluorescent dye 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide ( diS-C-3-[5]) revealed Chat, in contrast to valinomycin and mellitin, g lobomycin up to 30 mu M has no effect on the electrical transmembrane potential of S. melliferum. This indicates that the toxicity of globom ycin for spiroplasma cells is mainly if not exclusively owing to the i nhibition of spiralin processing. Added to previously published data, these results suggest that spiralin and probably other lipoproteins of mollicutes are acylated and membrane targeted by a mechanism involvin g notably the processing of the prelipoprotein precursor by a type II, globomycin-sensitive signal peptidase.