Abstract
Surface exposed proteins of vinlastine-sensitive human lymphoid cell line of leukemic origin (CCRF-CEM) were examined by lactoperoxidase catalyzed iodination and two-dimensional polyacrylamide gel electrophoresis methods. Spots which co-migrate with bovine brain tubulin were prodominently labeled in the whole membrane, but not in the high speed supernatant fraction of disrupted cells. The radiolabeled surface proteins were further identified as tubulin by peptide mapping and immunolocalization. Mild trypsinization of labeled cells removed the iodinated tubulin without affecting the protein staining pattern. Trypsinization also gave protection to the cells from the cytolytic effect of vinblastine and colchicine. The presence of surface exposed tubulin in this leukemic cell line suggests a possible mechanism for their enhanced sensitivity to the cytotoxic action of antimitotic drugs. Results from experiments to find the origin of the surface tubulin indicate that at least some of it comes from tubulin released into the medium. The specificity with which CEM tubulin binds to the cell surface and the fact that it is saturatable suggests a membrane "receptor" mechanism. The alpha component of the surface tubulin appears by radioautography to be an isomer of the alpha subunit in the pool tubulin. Iodination of normal human lymphocytes, NRK, L-929, and heart fibroblasts resulted in no labeling of the tubulin. Lymphoblasts isolated from patients with diagnosed leukemia do possess surface iodinatable tubulin. We demonstrate here the clear existence of tubulin on the surface of leukemic cells both in vitro and in vivo and suggest the possible source and therapeutic significance of this tubulin.