Binding of platelet glycoprotein Ibβ through the convex surface of leucine-rich repeats domain of glycoprotein IX

Background: The mechanism of assembly of the platelet glycoprotein (GP) Ib-IX complex from GPIb alpha, GPIb beta and GPIX subunits is not entirely clear. In this complex, ectodomains of both GPIb beta and GPIX subunits contain two leucine-rich repeats (LRR) and share high sequence similarity. However, they differ noticeably in stability, hampering further analysis of their interaction. Objectives and methods: Guided by analysis of the LRR structure, we report a well-folded Ib beta/IX chimera and its usage in dissecting GPIX function. Results: In this chimera, three non-contiguous sequences that may constitute the putative convex surface of the GPIb beta ectodomain are replaced by their GPIX counterparts. Like GPIb beta but unlike GPIX ectodomain, it can secrete from transfected Chinese hamster ovary cells and fold into a stable conformation. Furthermore, replacing the ectodomain in GPIX with the Ib beta/IX chimera, but not the GPIb beta ectodomain, preserved its interaction with GPIb beta as demonstrated by its native-like GPIb beta-induced increase in surface expression and coimmunoprecipitation. Conclusions: The putative convex surface of the LRR domain in GPIX is sufficient, in the context of full-length subunit, to mediate its association with GPIb beta.