Filamin A binding stabilizes nascent glycoprotein Ibalpha trafficking and thereby enhances its surface expression

The glycoprotein (Gp) Ib-IX-V complex is essential for platelet-mediated hemostasis and thrombosis. The cytoplasmic domain of its largest polypeptide subunit GpIbalpha possesses a binding region for filamin A, which links GpIb-IX-V to the platelet cytoskeleton. There is evidence that filamin A binding to GpIbalpha directs the surface expression of GpIb-IX. To investigate the mechanism of this effect, we examined GpIbalpha biosynthesis in Chinese hamster ovary (CHO) cells stably co-expressing wild-type or mutant GpIbalpha with GpIbbeta, GpIX with and without filamin A. We observed that surface GpIbalpha expression is enhanced in CHO cells co-expressing human filamin A. In comparison with cells expressing only GpIbalpha, GpIbbeta, and GpIX (CHO-GpIbalpha/betaIX), lysates from CHO-GpIbalpha/betaIX + filamin A-expressing cells showed greater amounts of immature, incompletely O-glycosylated and fully mature GpIbalpha, but lesser amounts of the approximately 15-kDa C-terminal peptide released when the extracellular domain of GpIbalpha is cleaved by proteases. When filamin A binding is eliminated by truncation of GpIbalpha at C-terminal residue 557 or by a deletion between amino acids 560-570, the decreased synthesis of mature GpIbalpha is accompanied by decreased immature GpIbalpha and by an increased immunodetectable C-terminal peptide. The synthesis of mature GpIbalpha in CHO-GpIbalpha/betaIX cells is eliminated by brefeldin A (which inhibits transport out of the endoplasmic reticulum (ER)) and restored by lactacystin (which inhibits proteasomal degradation). These results suggest that GpIbalpha binds to filamin A within the ER and that filamin A binding directs post-ER trafficking of GpIbalpha to the cell surface.