Endocytosis of glycophospholipid-anchored and transmembrane forms of CD4 by different endocytic pathways.

A glycophosphatidylinositol (GPI)-anchored form of the CD4 receptor was constructed by fusing the extracellular domain of CD4 to the COOH-terminus of decay accelerating factor (DAF), containing a signal for GPI-anchor attachment. The internalization of GPI-linked CD4 (CD4DAF) was compared to that of transmembrane CD4 using both [125I]gp120 and anti-CD4 antibodies. We show that transmembrane CD4 is rapidly endocytosed in transfected CHO cells, while CD4DAF is internalized at a rate approximately 3-fold slower. Immunoelectron microscopy suggests that whereas transmembrane CD4 is endocytosed via clathrin-coated vesicles, CD4DAF enters cells by an alternative pathway involving non-coated microinvaginations of the plasma membrane. Following internalization CD4DAF recycles through a primaquine-insensitive compartment, whereas the recycling of transmembrane CD4 is inhibited by primaquine, suggesting that the two receptors may recycle from distinct populations of early endosomes. Colocalization of both CD4DAF and CD4 with an antibody against a lysosomal membrane protein suggests that the two endocytic pathways may converge.