proteins on NuPage Bis-Tris 4-12% acrylamide gels (Invitrogen), transferred them to nitrocellulose membranes, and probed blots using primary polyclonal anti-Flag antibodies (Sigma-Aldrich) at a 1:2,000 dilution followed by secondary peroxidase-conjugated anti-rabbit antibodies (Jackson Immuno-Research Laboratories) at a 1:5,000 dilution. As a loading control, we also probed blots with polyclonal anti-GAPDH (glyceraldehyde 3-phosphate dehydrogenase; ProSci, Inc.). We visualized the immunoblots using a chemiluminescent assay (ECL; GE Healthcare).