Structural evidence of α‐aminoacylated lipoproteins of Staphylococcus aureus

Bacterial lipoproteins are known to be diacylated or triacylated and activate mammalian immune cells via Toll‐like receptor 2/6 or 2/1 heterodimer. Because the genomes of low G+C content Gram‐positive bacteria, such as Staphylococcus aureus, do not contain Escherichia coli‐type apolipoprotein N‐acyltransferase, an enzyme converting diacylated lipoproteins into triacylated forms, it has been widely believed that native lipoproteins of S. aureus are diacylated. However, we recently demonstrated that one lipoprotein SitC purified from S. aureus RN4220 strain was triacylated. Almost simultaneously, another group reported that another lipoprotein SA2202 purified from S. aureus SA113 strain was diacylated. The determination of exact lipidated structures of S. aureus lipoproteins is thus crucial for elucidating the molecular basis of host–microorganism interactions. Toward this purpose, we intensively used MS‐based analyses. Here, we demonstrate that SitC lipoprotein of S. aureus RN4220 strain has two lipoprotein lipase‐labile O‐esterified fatty acids and one lipoprotein lipase‐resistant fatty acid. Further MS/MS analysis of the lipoprotein lipase digest revealed that the lipoprotein lipase‐resistant fatty acid was acylated to α‐amino group of the N‐terminal cysteine residue of SitC. Triacylated forms of SitC with various length fatty acids were also confirmed in cell lysate of the RN4220 and Triton X‐114 phase in three other S. aureus strains, including SA113 strain and one Staphylococcus epidermidis strain. Moreover, four other major lipoproteins including SA2202 in S. aureus strains were identified as N‐acylated. These results strongly suggest that lipoproteins of S. aureus are mainly in the N‐acylated triacyl form.