Incorporation of radioactive tubulin into microtubules at steady state. Experimental and theoretical analysis of the effect of podophyllotoxin.

Addition of a substoichiometric amount of podophyllotoxin to microtubules which are at steady state decreases the rate of incorporation of radioactive subunits into the microtubules. The magnitude of this effect depends on the fraction of the total number of each of the microtubule ends which is capped by tubulin-podophyllotoxin subunits; these fractions depend on the concentration of podophyllotoxin used. A quantitative relationship is presented which correlates the extent of tubulin-podophyllotoxin capping of the microtubule ends with alterations in the rate for steady state subunit incorporation. An analysis based upon this relationship reveals that tubulin-podophyllotoxin caps both ends of the microtubule. Previously proposed mechanisms which involve tubulin-podophyllotoxin capping at only one end of the microtubule are analyzed.