Human profilin I and its interaction with membrane polyphosphoinositide lipids

Abstract

Human profilin 1 is a small (14kDa) cytoplasmic protein ubiquitously expressed in all cell types and necessary for normal cell growth and function in all eukaryotes. Profilin expression is significantly down-regulated in several cancers. Overexpression of exogenous profilin completely suppresses the tumorigenicity of mammary carcinoma cells. Several lines of evidence suggest that profilin may act as a tumour suppressor by blocking the PI3-Kinase/AKT pathway in some human cancers. In this study, giant unilamellular vesicles (GUVs) were used as a model to determine the interaction of profilin with membrane polyphosphoinositides (PPIs). A commercial confocal laser scanning microscope (Nikon C1) was used to obtain single point fluorescence correlation spectroscopy (FCS) and Number and Brightness data. From these data, the diffusion coefficient and fluorophore concentration was calculated. In order to perform these experiments, it was necessary to develop a method for immobilising the GUVs onto the bottom of the microscope chamber. Also, an S57C profilin mutant was labelled with a maleimide fluorophore. We found that the fluorescently labelled protein precipitated out of solution. To determine if the modified protein had the same three-dimensional solution structure as the native protein and investigate the possible cause of this precipitation, nuclear magnetic resonance (NMR) spectroscopy was performed. It was found that the mutant protein was fluorescently labelled at two cysteine residues. In addition to the mutated residue, an extra cysteine on the native protein sequence was also labelled. This finding may explain why the modified protein precipitates.