HELIX STABILITY IN BARSTAR PEPTIDES

The complex between the ribonuclease barnase and barstar, its intracel lular inhibitor, is a very good model for studying protein folding and molecular recognition. We have studied the stability of different pep tides that cover the barstar alpha-helix2 involved in the binding to b arnase. A linear correlation between the helical amphipathy of these p eptides and their inhibitory ability was obtained: the more helically amphipathic, the more the affinity for barnase. We estimated the amoun t of helix of these peptides in water and in trifluoroethanol by circu lar dichroism. There is a moderate correlation between the helical amp hipathy and the helical content in water, in agreement with previous r esults that have shown the importance of the hydrophobicity periodicit y in the design of peptides. The helical content in trifluoroethanol i s related to helical propensity and helical amphipathy, suggesting tha t the local sequence determines these maximum helicities. The predicte d helicity of these peptides, obtained using the algorithm AGADIR [Mun oz, V. & Serrano, L. (1994) Nat. Struct. Biol. l, 399-409], appears to correlate with their ability to inhibit the activity of barnase in wa ter. The correlation of inhibition constants, helical content in water , and maximum content of helix in trifluoroethanol with helical amphip athy supports the very important role of hydrophobicity pattern in pep tide stability.