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Signal peptidase (SPase) I is responsible for the cleavage of signal peptides of many secreted proteins in bacteria. Because of its unique physiological and biochemical properties, it serves as a potential target for development of novel antibacterial agents. In this study, we report the production, isolation, and structure determination of a family of structurally related novel lipoglycopeptides from a Streptomyces sp. as inhibitors of SPase I. Detailed spectroscopic analyses, including MS and NMR, revealed that these lipoglycopeptides share a common 14-membered cyclic peptide core, an acyclic tripeptide chain, and a deoxy-␣-mannose sugar, but differ in the degree of oxidation of the N-methylphenylglycine residue and the length and branching of the fatty acyl chain. Biochemical analysis demonstrated that these peptides are potent and competitive inhibitors of SPase I with K i 50 to 158 nM. In addition, they showed modest antibacterial activity against a panel of pathogenic Gram-positive and Gram-negative bacteria with minimal inhibitory concentration of 8 -64 M against Streptococcus pneumonniae and 4 -8 M against Escherichia coli. Notably, they mechanistically blocked the protein secretion in whole cells as demonstrated by inhibiting ␤-lactamase release from Staphylococcus aureus. Taken together, the present discovery of a family of novel lipoglycopeptides as potent inhibitors of bacterial SPase I may lead to the development of a novel class of broad-spectrum antibiotics.Proteins destined for secretion in both prokaryotic and eukaryotic organisms are initially synthesized as precursors with an amino-terminal extension known as signal (or leader) peptide. The signal sequence is removed by a signal peptidase (SPase) 1 that is localized in the cytoplasmic membrane in bacteria. Cleavage of precursors by SPase leads to the release of secreted proteins from the outer surface of cytoplasmic membrane. In bacteria, two major SPases, SPase I and SPase II with different cleavage specificities, have been identified. SPase I is responsible for processing the majority of secreted proteins (1-3), and SPase II is exclusively involved in processing glyceride-modified lipoproteins (4).SPase I is an attractive target for development of antibacterial agents because of its unique biochemical and physiological properties. It is essential for bacterial viability and growth as demonstrated by gene knockout and other genetic experiments (5-8). It is widely distributed in both Gram-positive and Gramnegative bacteria, as well as in Chlamydia. Genes encoding SPase I have been cloned and sequenced from different bacterial species, including many of clinically relevant bacteria (8, 9). The active domain of bacterial SPase I is exposed to the surface of cytoplasmic membrane as revealed by sequence and topological analysis (10 -12), and thus is accessible to potential inhibitors. In addition, SPases from bacteria and eukaryotic cells are different in composition, location, and possibly catalytic mechanism (13-17). These differences make it ...