The major proteins of the Escherichia coli outer cell-envelope membrane. Heterogeneity of protein I.

One of the major proteins of the Escherichia coli outer cell envelope membrane, protein I, can be separated electrophoretically into protein components Ia and Ib. Strain differences exist regarding presence or absence of component Ib and this component can selectively be lost by mutation to resistance against a phage. Both components Ia and Ib are further heterogeneous isoelectrically, and both together may contain at lease six separable isoelectric species. As judged by analysis of their cyanogen bromide fragments, Ia and Ib are almost identical concerning their primary structure; the difference (charge only or size and charge) was located in a part of the protein that does not correspond to the C-terminal or N-terminal regions. Components Ia and Ib thus represent essentially the same polypeptide and they may arise by a modification process in vitro or the existence of two almost identical genes.