Membrane interaction of 'peptide P' derived from the repeating motif of properdin

A 24 amino acid residue peptide corresponding to the central part of the 'thrombospondin-repeat' motif of the human serum protein properdin was synthesized. The peptide, termed 'peptide P', contains three tryptophans near the N-terminus and an arginine cluster close to the C-terminus. Its sequence closely matches a consensus sequence which has been claimed to characterize a sulfatide binding motif. Membrane binding of peptide P was analyzed using changes in its tryptophan emission upon adding small unilamellar vesicles. The peptide bound to the membranes in a way suggesting simple water/membrane partitioning. Analysis of electrostatic effects at different ionic strengths indicated small electrostatic contributions upon interaction with zwitterionic lipid, despite the large charge number (z = +4) of the peptide. Membrane affinity was increased by one order of magnitude if the bilayers contained 20% of negatively charged lipid. No difference could be detected whether the charged lipid was sulfatide or phosphatidylglycerol. Strong and rapid vesicle aggregation was evident as the peptide associated with the negatively charged vesicles. In addition, a fluorescent energy transfer assay with vesicles and internal total reflection fluorescence microscopy on supported bilayers were used to study membrane interaction of whole human properdin. No sulfatide specificity could be detected.