GPI-mediated anchoring or palmitoylation and/or myristoylation of proteins are biological modifications that allow the association of proteins with the inner and outer leaflet of plasma membrane microdomains.
Partitioning of proteins in cholesterol and sphingolipid enriched plasma membrane microdomains, called lipid rafts, is critical for many signal transduction and protein sorting events. Lipid-anchored proteins possess high affinity for lipid rafts and are currently exploited as markers to investigate fundamental mechanisms in protein sorting and signal transduction events.
Proteins with high lipid raft affinity are often glycosylphosphatidyl-inositol (GPI) anchored, cholesterol linked, or palmitoylated/myristoylated. Palmitoylation appears to increase lipid raft affinity but is not always sufficient for lipid raft association. GPI-anchored proteins probably partition into lipid rafts as a result of preferential packing of their saturated lipid anchors.
One interesting property of purified GPI-anchored proteins is their ability to re-insert into the plasma membrane upon incubation with a target cell, a process called Molecular Painting. Painted molecules remained stably associated with the cell surface for up to 12 h followed by a progressive decrease in surface expression upon successive cell division
Cell surface painting offers several advantages over conventional transfection/infection methods: virtually any cell type can be painted, including primary cells that are difficult to transfect, and most proteins of interest can be expressed as GPI-anchored fusion proteins. The painting efficiency is high and reveals homogenous membrane insertion of the GPI-anchored protein. Painting is performed under normal culture conditions with no detectable toxicity for the cells. Finally and most importantly, the introduced protein retains its biological function.