Evidence against a protein in plasma that is a product of a factor XI mRNA splice variant missing exons 6 and 7.

摘要

We read with interest the paper in Blood from Asselta et al describing a form of human factor XI (fXI) encoded by an mRNA splice variant lacking exons 6 and 7 (fXI-DELTA6/7). We observe 3 bands when plasma fXI purified by IgG affinity chromatography is size fractionated by nonreducing gel electrophoresis (Figure 1 A). The dominant 160 kDa form contains 2 identical subunits connected by a disulfide bond. The fainter band (70-80 kDa) migrating with reduced fXI represents single subunits. The third band, marked X in Figure 1 A, is likely the approximately 105-kDa species observed in plasma immunoprecipitates by Asselta et al. FXI subunits contain 4 apple domains (A1-A4, exons 3-10) and a protease domain (exons 11-15). AfXI-DELTA6/7 subunit would lack parts of A2 and A3 (amino acids 145-234). However, modeling (Figure 1B) indicates a complete apple domain could form from the N- and C-terminal portions of A2 and A3, respectively, encoded by exons 5 and 8. Asselta et al showed that fibroblasts transfected with fXI-DELTA6/7 cDNA synthesize a protein that may have this hybrid domain, and postulated that fXI-DELTA6/7 and the 105-kDa form in plasma are identical based on electrophoretic mobility. Here we show that the 105-kDa plasma form is not likely to be fXI-DELTA6/7.