Antibody directed against Friend leukemia virus stimulates DNA synthesis in a subpopulation of mouse B lymphocytes

Bacteriophage M13 procoat protein inserts into the plasma membrane as a loop structure

The major coat protein of bacteriophage M13 is synthesized as a precursor, the procoat, with a typical leader (signal) sequence of 23 residues at its NH2-terminus. A fusion protein that contains the NH2-terminal 141 residues of cytoplasmic ribulokinase and all but the first ten residues of M13 procoat was made. The fusion protein inserts into the plasma membrane of Escherichia coli and is processed by leader peptidase to give rise to a leader peptide of 155 residues and the mature coat protein of 50 residues. The NH2-terminus of the leader peptide remains in the cytoplasm and is protected from protease added to the medium outside of the cell. This indicates that M13 procoat inserts into the membrane as a loop structure and that the NH2-terminus of a leader peptide remains within the cytoplasm during membrane insertion.

Mitogenic effect of anti-Friend leukemia virus antiserum on T cells

It has been previously reported that goat- and rat antisera directed against Friend leukemia virus (aFLV) are mitogenic for some B cells, but not for T cells. Here we report that activation of T cells by concanavalin A (Con A) rendered T cells responsive to the mitogenic activity of aFLV. This activity was contained in the immunoglobulin fraction and could be absorbed by purified FLV preparations. Optimal conditions for measuring the mitogenc activity of aFLV include 48 h preincubation of thymocytes with 3 micrograms/ml Con A followed by reculturing the activated thymocytes for 28 h with aFLV. The acquisition of an aFLV-responsive state was dependent on early protein synthesis during the Con A-induced activation period. aFLV did not substitute for interleukin 2 (IL2) in a costimulator assay. Evidence is presented that aFLV acted in a cocultivation assay via growth factor(s). In contrast to control cultures, aFLV-treated lymphoblasts contained, in their supernatants, IL2 activity as demonstrated by their effect on an IL2-dependent T cell line. The data suggest that aFLV acted upon activated T cells by enhancing the endogenous production and/or release of IL2.