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

While lysozyme is a depolarizing chemorepellent in Tetrahymena, the entire lysozyme molecule is not necessary to activate the lysozyme receptor. Reduc ed lysozyme was cut into three fragments by cyanogen bromide cleavage and t he fragments (CB1, CB2 and CB3) were separated by HPLC. Behavioral bioassay s showed that the carboxy-terminal 24-amino-acid fragment, which we call CB 2, is 100 times more active than intact lysozyme as a chemorepellent. CB2 a ppears to activate the same receptor as lysozyme because behavioral cross-a daptation is seen between these two compounds and an antibody generated to the purified lysozyme receptor blocks responses to both lysozyme and CB2. T his is further supported by the observation that neomycin, which is a compe titive inhibitor of lysozyme binding, also inhibits CB2 responses. This inh ibition 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 elect rophysiological recordings documented that CB2 elicits a transient, depolar izing receptor potential that is similar to the lysozyme-induced depolariza tions except they are much smaller. CB2 is a more potent and specific ligan d for use in studies of the lysozyme receptor of Tetrahymena.