Preferential release of new outer membrane fragments by exponentially growing Escherichia coli

We have examined whether the outer membrane fragments released by normally growing Escherichia coli contain relatively old or new outer membrane. Double-label experiments show that after incorporation of radioactive leucine into E. coli protein, there is a preferential release of outer membrane material which contains a high percentage of newly labeled protein. This implies that outer membrane fragments are preferentially released from those regions where newly synthesized proteins are inserted into the outer membrane. We estimate that these insertion regions cover no more than 13% of the total outer membrane, and that newly inserted proteins diffuse in the plane of the outer membrane with a diffusion constant less than or equal to 5.10(-13) cm2/s.