The SPN antigen plays an essential role in mitosis, since microinjection of antibodies causes mitotic arrest. Here we show, by examination of the relative locations of SPN antigen, the centrosomal 5051 antigen and tubulin in normal mitotic, and in taxol-treated mitotic cells, that the SPN antigen is involved in organizing the microtubules of the spindle. The 210 kDa protein defined as SPN antigen relocates from the nuclear matrix to the centrosome at prophase, remains associated with the poles at metaphase and anaphase, and dissociates from the centrosomes in telophase. In taxol-treated mitotic cells, SPN staining shows a striking redistribution while 5051 antigen remains associated with centrosomes. SPN antigen is seen at the plasma membrane end of the rearranged microtubules. SPN antigen is always at the center of the multiple microtubule asters (5 to 20 per cell) induced by taxol, whereas 5051 again remains associated with the centrosomal complex (1 to 2 foci per cell). Microtubule nucleation is associated with the SPN antigen rather than with the 5051 antigen. Microinjection of SPN-3 antibody into taxol-treated mitotic PtK2 cells causes disruption of the asters as judged by tubulin staining of the same cells. Finally, SPN antigen extracted in soluble form from synchronized mitotic HeLa cells binds to, and sediments with, pig brain microtubules stabilized by taxol. This association of SPN antigen with microtubules is partially dissociated by 0.5 M NaCl but not by 5 mM ATP. Thus SPN antigen binds to microtubules in vitro and seems to act as a microtubular minus-end organizer in mitotic cells in vivo.

• © 1992 by Company of Biologists