The release of glycosylphosphatidylinositol-anchored proteins from the cell surface

Starting with the first description of the anchorage of a subset of cell surface proteins in eukaryotic cells from yeast to mammals with the aid of a glycosylphosphatidylinositol (GPI) moiety covalently attached to the carboxy-terminus of the protein, experimental evidence for the potential of GPI-anchored proteins (GPI-AP) of being released into the extracellular environment has been accumulating. GPI-AP are released as soluble monomers or multimers having lost their anchor or within hetero-/multimeric assemblies with their complete anchor remaining attached. The configurations reported so far for those assemblies encompass carrier protein-bound monomers, phospholipid- and cholesterol-harboring micelle-like complexes as well as membrane vesicles and particles. Each of these configurations prevents direct contact of the GPI anchor with the aqueous environment. Their structural diversity is reflected in the different molecular mechanisms underlying their release, which involve (i) proteolytic or lipolytic cleavage of the protein or GPI moiety, respectively, (ii) masking of the GPI anchor in the binding pocket of carrier proteins or in the phospholipid mono- or bilayers of particles or vesicles, respectively, and (iii) direct transfer of anchor-harboring GPI-AP from donor to acceptor cells through intimate contact of their plasma membranes. Release of GPI-AP may occur spontaneously or in response to certain endogenous or environmental stress signals and exert specific roles in the (patho)physiology of eukaryotic organisms which, however, are only incompletely understood so far.