APC down-regulates ATAP2 binding dependent on proteolytic activity and EPCR binding. (A) ATAP2 binding was quantified by ELISA after incubation with the indicated agonists in time course experiments. (B) Cells were incubated for 3 hours with the indicated concentrations of recombinant wild-type APC or proteolytically inactive APC S360A and ATAP2 binding was quantified. (C) Cells were preincubated (10 minutes) in the absence and presence of nonblocking (RCR-92) or blocking (RCR-252) anti-EPCR (25 μg/mL) followed by 3 hours of incubation with the indicated agonists and quantification of ATAP2 binding. Means plus or minus SEM are shown (n = 6 in panels A,C; n = 9 in panel B. **P < .005).