THE INTEGRIN ALPHA(IIB)-BETA(3), PLATELET GLYCOPROTEIN IIB-IIIA, CAN FORM A FUNCTIONALLY ACTIVE HETERODIMER COMPLEX WITHOUT THE CYSTEINE-RICH REPEATS OF THE BETA(3) SUBUNIT

Integrin alpha(IIb)-beta(3) binds fibrinogen via the recognition seque nce Arg-Gly-Asp-Ser (RGDS). We have used the baculovirus/insect cell e xpression system to study the structural requirements for the formatio n of a functionally active fragment of alpha(IIb)-beta(3). A tandem ba culovirus transfer vector was constructed containing the cDNA coding f or the heavy chain of human alpha(IIb) (alpha(IIbH), amino acids 1-874 ) and the cDNA coding for a truncated form of human beta(3) (t beta(3) ; amino acids 1-469). Sf9 insect cells were infected with the correspo nding baculovirus, and the produced soluble recombinant proteins were purified using an RGD-like affinity column. The bound receptor fragmen ts were specifically eluted with RGDS and existed as a heterodimeric c omplex (rec alpha(IIbH)-t beta(3)) with an apparent M(r) of 160,000, I n an immunocapture assay, the monoclonal antibody pl-55, which only re cognizes the functionally active form of alpha(IIb)-beta(3), bound to the purified complex. Rec alpha(IIbH)-t beta(3) specifically bound I-1 25-fibrinogen with an affinity comparable with that of purified platel et alpha(IIb)-beta(3). Electron micrographs of rotary-shadowed rec alp ha(IIb)-B-3 showed that the complex had the characteristic globular he ad, but the two rodlike tails were 4-6 nm shorter than those found in intact alpha(IIb)-beta(3). Thus, the cysteine-rich repeats of beta(3) are not required for assembly, stability, and functional activity of t his integrin.