Interaction between two major outer membrane proteins of Escherichia coli: the matrix protein and the lipoprotein

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Abstract
The affinity of the matrix protein, one of the major outer membrane proteins of E. coli, for the peptidoglycan was examined by extracting the cell envelope complex at 55.degree. C with 2% sodium dodecyl sulfate containing different amounts of NaCl. The matrix protein was extracted from the peptidoglycan of a mutant strain (lpo) that lacks another major membrane protein, the lipoprotein, at a lower NaCl concentration than was the matrix protein of the wild-type cell (lpo+). When the envelope fraction of the wild-type strain was treated with trypsin, which is known to cleave the bound-form lipoprotein from the peptidoglycan, the affinity of the matrix protein for the peptidoglycan decreased to the same level as that of the affinity of the matrix protein for the peptidoglycan of the mutant strain. The free-form lipoprotein was also retained in the matrix protein-peptidoglycan complex, although the extent of retention of the free form of the lipoprotein was less than that of the matrix protein. Apparently, both the free and the bound forms of the lipoprotein are closely associated with the matrix protein and the bound form of the lipoprotein plays an important role in the association between matrix protein and the peptidoglycan.