Recognition of the carboxyl-terminal signal for GPI modification requires translocation of its hydrophobic domain across the ER membrane

Abstract

A carboxyl-terminal hydrophobic domain is an essential component of the processed signal for attachment of the glycosyl-phosphatidylinositol (GPI) membrane anchor to proteins and it is linked to the site (ω) of GPI modification by a spacer domain. This study was designed to test the hypothesis that the hydrophobic domain interacts with the lipid bilayer of the endoplasmic reticulum (ER) membrane to optimally position the ω site for GPI modification. The hydrophobic domain of the GPI signal in the human folate receptor (FR) type α was substituted with the carboxyl-terminal segment of the low-density lipoprotein receptor (LDLR), including its membrane spanning region, without altering either the spacer or the ω site. The FR-α/LDLR chimera was not GPI modified but was attached to the plasma membrane by a polypeptide anchor. When the carboxyl-terminal half of the hydrophobic transmembrane polypeptide in the FR-α/LDLR chimera was altered by introduction of negatively charged (Asp) residues, or when the cytosolic domain in the chimera was deleted, the mutated proteins became GPI-anchored. On the other hand, attachment of a carboxyl-terminal segment of LDLR including the entire cytosolic domain to FR-α converted it into a transmembrane protein. The results indicate that in the FR-α/LDLR chimera the inability of the cellular machinery for GPI modification to recognize the hydrophobic domain is not due to the intrinsic nature of the peptide, but is rather due to the retention of the peptide within the lipid bilayer. It follows that the hydrophobic domain in the signal for GPI modification must traverse the ER membrane prior to recognition of the ω site by the GPI-protein transamidase. The results thus establish a critical topographical requirement for recognition of the GPI signal in the ER.