Outer-membrane vesicles released by normally growing Escherichia coli contain very little lipoprotein.

The lipoprotein content of the outer-membrane medium vesicles, which are released from Escherichia coli during normal growth, was compared to the lipoprotein content of the corresponding cellular outer membranes. It was found that the medium vesicles contained only 35% free lipoprotein and almost none of the bound lipoprotein when compared with cellular outer membranes. Medium vesicles also had reduced amounts of protein II and a protein V (Mr = 16 000), while they contained large amounts of pore-forming proteins I and lamB. A mechanism is proposed in which outer membrane vesicles are formed when the outer membrane expands faster than the underlying peptidoglycan layer. The lack or enrichment of individual proteins in medium vesicles may be determined by their interactions with the peptidoglycan-bound lipoprotein complex.