Identification of vimentin as an endothelial protein cofactor in SN-APS patients. (A) Endothelial cell-surface membrane proteins separated by 2DE were transferred onto nitrocellulose membrane and analyzed by immunoblotting with serum from 2 APS-seronegative patients. Two spots, with a molecular weights of 54 and 57 kDa, strongly reactive with serum IgG were identified (circled). (B, C) The 2 spots identified were excised from 2DE gel, digested with trypsin, and then analyzed by MALDI time-of-flight mass spectrometry. MS/MS spectra of trypting peptides matching 2 isoforms of vimentin. x-axis, m/z; y-axis, relative ion intensity. The matched amino acid sequences are bold underlined.