N-terminal 14 amino acids of GPIbβ and surface expression of GPIX.

The N-terminal 14 amino acids of GPIbβ are essential for the surface expression of GPIX. The chimeras GPIX22/β15, GPIX36/β32, and GPIX53/49 were cotransfected with GPIX into HEK293T cells and reacted with the anti-GPIX antibody FMC 25 (panels A-D) or the complex-specific antibody AK1 (panels E-H). Panels A and E are wildtype beta and wildtype IX. Panels B and F are the chimera GPIX22/β15, which brings GPIX to the surface and is complexed to GPIX22/β15 since there is a significant increase in surface fluorescence in these cells when reacted with the antibody AK1 as compared with mock-transfected cells. In contrast, when cells simultaneously transfected with chimeras GPIX36/β32 (panels C, G) and GPIX53/49 (panels D, H) and GPIX are reacted with either the anti-GPIX (panels C, D) antibody or AK1 (panels G, H), there is no increase in fluorescence compared with mock-transfected cells. Thus, the N-terminal 14 residues of GPIbβ are essential for the surface expression of GPIX.