A defined fragment of bacterial protein I (OmpF) is a polyclonal B-cell activator.

Protein I from the outer membrane of Escherichia coli and other members of the family Enterobacteriaceae is a potent mitogen and polyclonal B-lymphocyte activator. To determine the part of the polypeptide responsible for biological activity, we cleaved the molecule into defined polypeptide fragments of approximate molecular weights 24,000, 15,000, 9,000, 7,000, and 3,000 by using the cyanogen bromide method. The fragments were purified by gel permeation chromatography and by preparative polyacrylamide gel electrophoresis. They were investigated for mitogenicity and for the induction of immunoglobulin synthesis in lymphocyte cultures from several inbred mouse strains. The fragment of molecular weight 24,000 turned out to be a potent polyclonal B-lymphocyte activator comparable to native protein I. The low-molecular-weight fragments exhibited only marginal effects. Neither purified T lymphocytes nor thymocytes were activated. Our results show that a defined fragment of protein I is responsible for its lymphocyte-stimulating activity.