Experimental determinations of tubulin in the in vivo mitotic apparatus of sea urchin zygotes.

Mitotic apparatus (MA) were isolated from zygotes of a sea urchin (Strongylocentrotus purpuratus), using hexylene glycol (pH 6.4) as lysing-stabilizing agent. Protein was measured in the MA pellet and in the remainder of the cell lysate (using the Lowry procedure). Tubulin was measured in the MA pellet and in the remainder of the cell lysate (using microdensitometry of stained gels after sodium dodecyl sulphate - polyacrylamide gel electrophoresis). From these data we calculated the maximum possible amounts of tubulin in the isolated MA and in the MA in vivo; in these calculations we assumed that all the tubulin in the cell is associated with the MA, and we assumed that, as reported in the literature, the MA lose 90% of their dry matter during the isolation. We conclude that tubulin probably comprises less than 7% of the protein in the in vivo MA, and, even if there are very large errors, tubulin is considerably less than half the protein in the MA.