The isolation and characterisation of cytoplasmic and outer membranes from Micrococcus cryophilus

Abstract

The cytoplasmic and outer membranes of the Gram-negative bacterium, Micrococcus cryophilus, have been separated and purified. Both membrane preparations consist of a mixture of closed and apparently open vesicles, which vary in size but those of the outer membrane are on average 1.5 times the diameter of those of the cytoplasmic membrane. The membranes were characterised by their 2-keto-3-deoxyoctonate content and the activity of marker enzymes. There are gross differences in the protein and phospholipid composition of the two membranes. The outer membrane contains three major polypeptides, whereas the cytoplasmic membrane has many more. In addition the outer membrane is enriched in cardiolipin, at the expense of phosphatidylglycerol and phosphatidylethanolamine, relative to the cytoplasmic membrane. There are no significant differences in the fatty acid composition of the phospholipid classes, but all phospholipids of the outer membrane were slightly more saturated than those of the cytoplasmic membrane. Wax esters are present in both cytoplasmic and outer membranes. The significance of these findings is discussed in relation to known differences in the fluidity of the cytoplasmic and outer membranes in M. cryophilus and the specialized roles these membranes play.