mplified by PCR with primers that included a BamHI site at the 59 end and a NotI site at the 39 end of the A2 domain. The amplified fragment was cloned by TA-cloning into the vector pCR2.1-TOPO. Mutations in the VWF A2 domain were introduced into the wild-type fragment according to the protocol of the QuikChange site-directed mutagenesis kit. The sequence of the wild-type and mutant VWF A2 fragments were confirmed by DNA sequencing. Inserts containing the wild-type and mutant VWF A2 fragments were excised from the pCR2.1-TOPO vector by BamHI and NotI digestion and WP1130 site inserted into the corresponding sites in the pNBiosecPC4 expression vector. In this expression system, recombinant proteins were secreted with dual tags: an N-terminal biotin-tag and a C-terminal protein C epitope tag. Expression of recombinant VWF A2 fragment. Expression plasmids were transfected into HEK293 Tet-On cells with lipofectamine 2000. Transient expression was induced by doxycycline in the presence of biotin in FreeStyle 293 serum-free culture medium for 72 hr as previously described. Culture medium containing secreted recombinant VWF A2 fragments was desalted over Sephadex G25 into 10 mM Hepes pH 7.4, 2 mM CaCl2 to remove unincorporated biotin. A mixture of protease inhibitors was added to the recombinant fragments and stored at 280u C before use. Expression of recombinant ADAMTS13. Recombinant ADAMTS13 was expressed with a C-terminal biotin tag with the pCBioSec vector in stably transfected HEK293 Tet-On cells as previously described. Serum-free FreeStyle 293 medium containing recombinant ADAMTS13 was concentrated tenfold by centrifugation in an Ultracel 10K centrifugal filter and desalted over Sephadex G-25 to remove biotin and low molecular weight molecules. The concentrated recombinant ADAMTS13 preparation was treated with a mixture of protease inhibitors and stored at Structural Basis of Type 2A VWD 280uC. Recombinant ADAMTS13 was quantified in western blots probed with streptavidin-HRP by comparison to serial dilutions of a reference preparation of biotinylated albumin, which contains 4 moles of biotin per mole of albumin, prepared by the chemical biotinylating agent ChromaLink. The extent of biotinylation with ChromaLink was determined by absorption spectroscopy of a chromophore 
in the biotin linker. Recombinant ADAMTS13 was used without further purification. ADAMTS13 cleavage assays. The rates of cleavage of recombinant A2 fragments were measured by incubating 20 ng of each recombinant A2 fragment with 4 ng of recombinant ADAMTS13 in 10 mM Hepes, pH 7.2, 2 mM CaCl2, with or without urea, at 37uC for varying amount of time. For PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22211113 A2 fragments that were cleaved slowly, cleavage reactions were stopped by EDTA at 0, 10, 20, 30, 60 and 120 min. For A2 fragments that were cleaved rapidly, cleavage reactions were stopped by EDTA at 0, 5, 10, 20 and 30 min. The extent of A2 fragment cleavage at each time point was determined by SDSPAGE and western blotting and was expressed as percent of the A2 fragment cleaved. The rate of cleavage was expressed as the percent of A2 fragment cleaved per minute. In cases when the rates of cleavage were fast and became nonlinear with time, initial rates extrapolated to time 0 were used for comparison. In the SDSPAGE and western blot analyses, the reaction mixtures were reduced and fractionated by SDS-PAGE on 420% gradient polyacrylamide gels after reduction. The fractionated products were transferred onto nitrocellulose membranes, blo