In vivo and in vitro characterization of Staphylococcus aureus and Bacillus subtilis polyglycerolphosphate lipoteichoic acid synthases

Staphylococcus aureus lipoteichoic acid (LTA) consists of a 1,3-linked polyglycerolphosphate chain retained in the bacterial membrane by a glycolipid anchor. The LTA backbone is produced by the lipoteichoic acid synthase LtaS, a membrane protein with five transmembrane helices and a large extracellular enzymatic domain (eLtaS). Proteomic studies revealed that LtaS is efficiently cleaved, and here it was demonstrated that the eLtaS domain is released into the culture supernatant as well as partially retained within the cell wall fraction. However, using an in vivo LtaS activity assay, it was shown that only the full-length LtaS enzyme is able to synthesize LTA. Neither expression of a secreted eLtaS variant, created by replacing the N-terminal membrane domain with a conventional signal sequence, nor expression of eLtaS fused to a single or multi-transmembrane domains of other staphylococcal proteins resulted in the production of LTA. These data indicate that the transmembrane domain of LtaS play an essential, yet unknown, role in LtaS enzyme function. In addition, the protease responsible for LtaS cleavage was identified. It was found that a S. aureus strain in which the gene encoding for the essential signal peptidase SpsB was cloned under inducible expression control showed an accumulation of the full-length LtaS enzyme in the absence of the inducer. These data suggest that SpsB is involved in LtaS cleavage. Four LtaS orthologues, YflE, YfnI, YqgS and YvgJ, are present in Bacillus subtilis. Using an in vitro enzyme assay and purified protein, it was determined that all four B. subtilis proteins are Mn2+-dependent metal enzymes that use the lipid phosphatidylglycerol as substrate. It was shown that YflE, YfnI and YqgS are bonafide LTA synthases capable of producing polyglycerolphosphate chains, while YvgJ appears to function as an LTA primase, as indicated by the accumulation of a glycolipid with the expected chromatographic mobility of GroP-Glc2-DAG. Taken together, experimental evidence for the enzyme function of all four B. subtilis LtaStype proteins is provided in this work and it was shown that all four enzymes are involved in the LTA synthesis process