A Novel Interaction Linking the FAS-II and Phthiocerol Dimycocerosate (PDIM) Biosynthetic Pathways

Abstract

The fatty acid biosynthesis (FAS-II) pathway in Mycobacterium tuberculosis generates long chain fatty acids that serve as the precursors to mycolic acids, essential components of the mycobacterial cell wall. Enzymes in the FAS-II pathway are thought to form one or more noncovalent multi-enzyme complexes within the cell, and a bacterial two-hybrid screen was used to search for missing components of the pathway and to furnish additional data on interactions involving these enzymes in vivo. Using the FAS-II ␤-ketoacyl synthase, KasA, as bait, an extensive bacterial two-hybrid screen of a M. tuberculosis genome fragment library unexpectedly revealed a novel interaction between KasA and PpsB as well as PpsD, two polyketide modules involved in the biosynthesis of the virulence lipid phthiocerol dimycocerosate (PDIM). Sequence analysis revealed that KasA interacts with PpsB and PpsD in the region of the acyl carrier domain of each protein, raising the possibility that lipids could be transferred between the FAS-II and PDIM biosynthetic pathways. Subsequent studies utilizing purified proteins and radiolabeled lipids revealed that fatty acids loaded onto PpsB were transferred to KasA and also incorporated into long chain fatty acids synthesized using a Mycobacterium smegmatis lysate. These data suggest that in addition to producing PDIMs, the growing phthiocerol product can also be shuttled into the FAS-II pathway via KasA as an entry point for further elongation. Interactions between these biosynthetic pathways may exist as a simple means to increase mycobacterial lipid diversity, enhancing functionality and the overall complexity of the cell wall.

Chapter 17 Bacterial Fatty Acid Synthesis and its Relationships with Polyketide Synthetic Pathways

“…In mycobacteria the FAS II 3-ketoacyl-ACP synthase (KasA) has been found to interact with PpsB and PpsD, two polyketide modules involved in the biosynthesis of the virulence lipid, phthiocerol in E. coli (Kruh et al, 2008). The interaction with PpsB, a four-module PKS protein containing an ACP domain, was examined in detail.…”

“…The reverse transfer did not occur. The PKS to FAS transfer was proposed to serve to increase the diversity of mycobacterial lipids (Kruh et al, 2008). …”

Bacterial lipids: Metabolism and membrane homeostasis

“…Every FabH catalyzes a Claisin-type condensation reaction between an acyl-CoA substrate and malonyl-ACP using a bi-bi ping-pong mechanism [30]. The acyl-CoA binds to the enzyme and the acyl group is inserted into the hydrophobic active site tunnel of the protein [25,26,31].…”

The Molecular Genetics of Mycolic Acid Biosynthesis

“…This raises the possibility that lipids could be transferred between the two pathways as a means of increasing mycobacterial lipid diversity (78).…”