Characteristics of the release of the surface coat protein from bloodstream forms of Trypanosoma brucei.

Bloodstream forms of the African trypanosomes undergo antigenic variation in their mammalian host. This process involves removal of the existing variant coat protein and its replacement with another. The mechanism by which the surface coat protein is released to the external supporting medium has been shown to depend in vitro specifically on the presence of calcium ions together with the calcium ionophore. A-23187, and to be inhibited by Zn2+. Release of the surface coat protein was not stimulated by conditions designed to alter the plasma membrane potential or the major ionic gradients across that membrane. Release could be stimulated by inhibiting the energy metabolism of these glycolysing cells with 2-deoxyglucose, which probably prevents the energy-dependent mechanisms that normally keep the cytoplasmic Ca2+ concentration low. These results and the finding that the release process was strongly temperature dependent suggested the possible mediation of some as yet undefined enzymatic reaction.