Chemosensory responses of Tetrahymena thermophila to CB2, a 24-amino-acid fragment of lysozyme.

Abstract

While lysozyme is a depolarizing chemorepellent in Tetrahymena, the entire lysozyme molecule is not necessary to activate the lysozyme receptor. Reduced lysozyme was cut into three fragments by cyanogen bromide cleavage and the fragments (CB1, CB2 and CB3) were separated by HPLC. Behavioral bioassays showed that the carboxy-terminal 24-amino-acid fragment, which we call CB2, is 100 times more active than intact lysozyme as a chemorepellent. CB2 appears to activate the same receptor as lysozyme because behavioral cross-adaptation is seen between these two compounds and an antibody generated to the purified lysozyme receptor blocks responses to both lysozyme and CB2. This is further supported by the observation that neomycin, which is a competitive inhibitor of lysozyme binding, also inhibits CB2 responses. This inhibition may be due to the fact that neomycin is highly positively charged (+5 at pH 7.0) and CB2 has a net charge of +4 at pH 7.0. Intracellular electrophysiological recordings documented that CB2 elicits a transient, depolarizing receptor potential that is similar to the lysozyme-induced depolarizations except they are much smaller. CB2 is a more potent and specific ligand for use in studies of the lysozyme receptor of Tetrahymena.