Heterogeneous diffusion of a membrane-bound pHLIP peptide.

Lateral diffusion of cell membrane constituents is a prerequisite for many biological functions. However, the diffusivity (or mobility) of a membrane-bound species can be influenced by many factors. To provide a better understanding of how the conformation and location of a membrane-bound biological molecule affect its mobility, herein we study the diffusion properties of a pH low insertion peptide (pHLIP) in model membranes using fluorescence correlation spectroscopy. It is found that when the pHLIP peptide is located on the membrane surface, its lateral diffusion is characterized by a distribution of diffusion times, the characteristic of which depends on the peptide/lipid ratio. Whereas, under conditions where pHLIP adopts a well-defined transmembrane alpha-helical conformation the peptide still exhibits heterogeneous diffusion, the distribution of diffusion times is found to be independent of the peptide/lipid ratio. Taken together, these results indicate that the mobility of a membrane-bound species is sensitive to its conformation and location and that diffusion measurement could provide useful information regarding the conformational distribution of membrane-bound peptides. Furthermore, the observation that the mobility of a membrane-bound species depends on its concentration may have important implications for diffusion-controlled reactions taking place in membranes.