Role of the cytoplasmic portion of CD34 in tyrosine phosphorylation. (A) Amino acid sequence of the cytoplasmic portions of the full-length CD34 (CD34^full; top) and the truncated CD34 (CD34^trunc; bottom). Vertical bars indicate conserved residues. PTK, a candidate tyrosine phosphorylation site; PKC, a consensus phosphorylation site for protein kinase C. (B) FACS analysis of U937 cells stably transfected with vector alone, CD34^full, or CD34^trunc. Cells were stained with anti-CD34 (4A1, solid lines) or with the secondary reagent alone (dotted lines). (C) Western blot of Triton X-100 lysates from U937 cells stably transfected with Vector alone (lane 1), CD34^full (lane 2), or CD34^trunc (lane 3), probed with anti-CD34 (4A1). (D) Western blots of equal amounts of Triton X-100 lysates from U937 cells stably transfected with vector, CD34^full, or CD34^trunc, probed with anti-pTyr (4G10). Cells were stimulated for 30 minutes with anti-CD34 (4A1) or MOPC21 (control) as described in the legend to Fig 1. Molecular size markers are indicated on the left (kD).