Characterization of two Haemophilus somnus Fc receptors

Haemophilus somnus expresses two types of receptors that bind to the Fc region of bovine IgG, IgA and IgM. In this study, the relationship between these two types of Fc receptors is characterized. The high molecular mass receptors (350, 270 and 120 kDa) were secreted into the culture medium and were also in the insoluble protein fraction of the culture medium. The 41 kDa Fc receptor, which is a major outer-membrane protein, was only present in the insoluble protein fraction. Peptide mapping of the two types of Fc receptors suggests that the 41 kDa receptor is related to the high molecular mass receptor complex. Disulphide linkage is unlikely to be the mechanism of association of the 41 kDa receptor with the high molecular mass receptors since reducing agents had no effect on separating the individual receptors. Although the 41 kDa receptor is a major protein in the outer membrane of H. somnus, it does not react with convalescent bovine sera in Western blots. In contrast, convalescent bovine sera reacts intensely with the high molecular mass receptors in Western blots.