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The fast reaction of the actin-based cytoskeleton in motile cells after stimulation with a chemoattractant requires a signal-transduction chain that creates a very specific environment at distinct regions beneath the plasma membrane. Dictyostelium hisactophilin, a unique actin-binding protein, is a submembranous pH sensor that signals slight changes of the H+ concentration to actin by inducing actin polymerization and binding to microfilaments only at pH values below seven. It has a relative molecular mass of 13.5K and its most unusual feature is the presence of 31 histidine residues among its total of 118 amino acids. The transduction of an external signal from the plasma membrane to the cytoskeleton is poorly understood. Here we report the protein's structure in solution determined by nuclear magnetic resonance spectroscopy. The nuclear Overhauser effect intensities of the three-dimensional nuclear Overhauser spectra were used directly in the calculations. The overall folding of histactophilin is similar to that of interleukin-1 beta and fibroblast growth factor, but the primary amino-acid sequence of hisactophilin is unrelated to these two proteins.

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A general route to azadepsipeptides, a new class of pseudopeptides, has been established. The methodology was applied to the synthesis of a bis-aza analogue of the antiparasitic cyclooctadepsipeptide PF1022A. Comparison of the X-ray crystal structures of natural PF1022A (8) and the chimeric aza analogue 9 revealed that the introduction of nitrogen in the backbone of PF1022A results in almost complete conservation of the 3D structure with only minor deviations at the new nitrogen positions.