Molecular genetics of membrane phospholipid synthesis.

I have attempted to illustrate the genetic and biochemical complexity of membrane-lipid synthesis by focusing, primarily, on E. coli. The use of molecular genetics to probe membrane lipids is relatively new. Many important questions of phospholipid biochemistry remain unanswered. In the coming years our growing knowledge of the molecular genetics of phospholipids must be applied to the solution of the following problems: How does a cell regulate its total phospholipid content in relationship to macromolecules, especially membrane proteins, cell wall components, and nucleic acids? Why do E. coli and Caulobacter behave differently in this respect? How does a cell regulate its characteristic ratios of polar headgroups and fatty acyl chains? Why does overproduction of phosphatidylserine synthase have no effect on phospholipid composition? How is lipid topography established, both in terms of intramembrane movement (flip-flop) and intermembrane movement? Are there transport systems (flippases) for short-chain diacylglycerophospholipids in E. coli, as in mammalian microsomes, and can flippase mutants be isolated? What are the functions of the many individual phospholipid species? Does E. coli have a functional equivalent of the mammalian phosphatidylinositol cycle? A complete set of phospholipid mutants, together with phenotypic suppressors, should help to answer these questions by allowing selective perturbations in vivo and physiological studies of associated phenotypes. In addition, molecular cloning is already providing access to large quantities of the lipid gene products, opening the door to biophysical and chemical studies of lipid-protein interactions. A unique feature of genetics, as applied to complex biochemical or physiological systems, is the high frequency of unanticipated discoveries that accompany the characterization of new mutants. In our work, this is best illustrated by the analysis of phosphatidylglycerol-deficient mutants of E. coli, which provided the clue (i.e. lipid X) that permitted the elucidation of lipid A biosynthesis. The interconnection of metabolic pathways and important control mechanisms are often revealed by the study of mutants. In the case of E. coli it is best to consider the many lipids and proteins of the envelope as a whole. Considering how few mutant alleles are available for the lipid genes of E. coli, it will be important to create many more genetic lesions in order to gain a full understanding of regulation and function.