The “fluid mosaic” model updated: a “patchwork”

1. The “fluid mosaic” model updated: a “patchwork” • Composition =10 - 25% of dry weight of cell ~30% of membrane is lipid ~70% is protein • ~70 - 80 Å (7 - 8 nm) thick, like eukaryotic cell membrane • proteins are asymmetrical • integral vs. peripheral Engelman; Nature 438:578 (2005)

2. ester linkage between glycerol and fatty acid (FA) glycerol moiety General phospholipid (PL) structure • Bacterial phospholipids (PLs): • Similar to eukaryotic phospholipids except the side chains differ • phospholipid fatty acids (PLFA) can be C14 - C20 (C16 - C18 common) • composition is affected by temperature, growth conditions

3. Fatty acid variations Phosphatidic acid R1 is commonly C16:0 R2 is commonly C16:1 or C18:1

4. Cardiolipin (CL) Polar head groups • Side group chemistry affects charge, protein translocation, DNA interactions, extrcellular signal sensing, carbohyadrate interactions

5. Examples of PL head group compositions PE: phosphatidyl ethanolamine; PG: phosphatidyl glycerol; DPG: diphosphatidyl glycerol (CL; cardiolipin)

6. Modification of polar head group mutations in conversion of PS to PE are lethal in E. coli proportions of PG and CL depend on growth phase: PG is higher in log phase; CL is higher in stationary phase Curr. Opinion Microbiol. 8:135 (2005)

7. Phospholipids can be unevenly distributed Some evidence for heterogeneous (patchy) distribution of some phospholipids in the membrane: Phosphatidylethanolamine (PE) is found at the mid-point (septa) of Bacillus but evenly distributed in Gram-negative cells May be involved in cell division and sporulation Mol. Microbiol. 61:1110 (2006)

8. Two pathways are described for the synthesis of PC in bacteria . In the methylation pathway, the precursor phospholipid phosphatidylethoanoletamine (PE) is methylated in three consecutive steps by an enzyme named phospholipid N-methyltransferase (Pmt), using S-adenosylmethionine (SAM) as a methyl donor. In the Pcs pathway the enzyme phosphatidylcholine synthase (Pcs) synthesizes PC using choline and CDP-diacylglycerol (DAG) as substrates.

9. Fatty acid synthesis CO2 CoA malonyl~CoA acetyl~CoA H3C-CO~CoA HOOC-CH2-CO~CoA malonyl~ACP ACC (acetylCoA-carboxylase, 4 subunits) acyl carrier protein (ACP) Condensing enzyme (FabH) Enoyl- Ketoacyl- Preferentially produces C16 and C18 lengths because of enzyme preference for substrate Hydroxyacyl

11. LPA (lysophosphatidic acid)

12. Fatty acids synthesis is a target for antibiotics

14. New antimicrobial - Platensimycin -ketoacyl-ACP-synthase (FabF) http://www-jmg.ch.cam.ac.uk

17. Lipoprotein synthesis is coupled to flipping of 2 acyl-GPE The 2-acyl-GPE cycle and domain structures of proteins expressed in operons containing LplT family members. A topological diagram of the proteins involved in the 2-acyl- GPE cycle in E. coli. External 2-acyl-GPE is generated by the transacylationof the 1-position fatty acid from PtdEtn to the N terminus of the major outer membrane lipoprotein (Lpp) catalyzed by the product of thelnt gene. The resultant 2-acyl-GPE is transported to the cytosolic side of the inner membrane by LplT (YgeD) where it is acylated by the bifunctional Aas utilizing acyl-ACP as the acyl donor. Aas either acquires acyl-ACP from the biosynthetic pathway or activates a free fatty acid with ATP-Mg2 and transfers it to a bound ACP subunit to form acyl-ACP. The inner membrane ABC transporter, MsbA, is implicated in the ATP-dependent efflux of lipid A and PtdEtn from the inner to the outer aspect of the membrane. J Biol Chem. 2005 Mar 25;280(12):12028-34.

18. Fatty acids and membrane fluidity Branched fatty acids

19. FabA catalyzes the key step in the synthesis of unsaturated fatty acids (UFA)