1 Biomolecular Research Institute, 343 Royal Parade, Parkville VIC 3052
2 CSIRO Division of Biomolecular Engineering 343 Royal Parade, Parkville VIC 3052
Membrane-active amino-acid sequences are widespread and include insect and vertebrate toxins such as melittin and magainin and fusion and cytolytic sequences of enveloped viruses such as the myxo- and paramyxoviruses and retroviruses such as the human immunodeficiency virus (HIV). Short peptide inhibitors have been made that inhibit a number of these sequences, although their mode of action, for the most part, remains unclear. The inhibition of the cytolytic action of melittin provides a good model to study this mode of action because the mechanism of melittin interaction with membranes is fairly well understood.
There are three ways in which an inhibitor might work. One is by stabilising all the membranes of all the potential target cells against perturbation by the agent. The other two mechanisms both require that the inhibitor combine specifically with the sequence changing its conformation so that it either can no longer interact with the membrane or if it does interact it does so as a non-perturbing complex. From the point of view of designing practical antagonists, the latter two mechanisms are preferable. Given a high enough binding constant the effective concentration of the inhibitor need only be sufficient to saturate the agent present in the tissues. In the case of the first mechanism, however, it might be necessary to provide enough stabiliser for every cell in the body.
We have examined the action of an inhibitor of melittin cytolysis (IVIFDC) using spin-labelling, UV flourescence spectrometry and molecular modelling combined with studies of the effects on inhibition of variations in the sequence of the inhibitor. We have concluded that the inhibitor works by complexing specifically with the melittin, preventing it from entering the bilayer.
Our findings are compared with those for other membrane-active peptide/inhibitor systems, particularly the fusogenic and cytolytic sequences of of the transmembrane glycoprotein of
HIV-1.