Fig. 1.

PIM biosynthesis and inositol acylation. A: Biosynthesis of AcPIM4, AcPIM5, and AcPIM6 are dependent on polyprenol-phosphate-mannose (PPM) as a mannose donor (17), suggesting that these steps take place in the periplasmic leaflet. For the biosynthetic steps between AcPIM2 and AcPIM6, PimE is the only known mannosyltransferase (16). Both AcPIM2 and AcPIM6 are readily available for inositol acylation reaction (19), implying that they are on the same leaflet. Based on these observations, we speculate that AcPIM2 flops from the cytoplasmic leaflet to the periplasmic leaflet (see main text for detailed discussion). B: Inositol acylation induced by membrane fluidization stress (19). The fatty acid (shown in red) is esterified at the 3-OH of the myo-inositol ring (23).