AbstractLimited proteolysis of tubulin with subtilisin results in cleavage of both the α and β subunits, releasing small peptides from the C‐terminal ends. At 37°C the digested tubulin assembles into polymorphic structures: microtubules with attached ribbons in the presence of GTP, rings in the presence of GDP, and protofilament spirals in the presence of vinblastine. Undigested tubulin does not assemble under these conditions. Rings and Vinca‐induced spiral structures are assembled from undigested tubulin only when microtubule‐associated proteins, high Mg2+concentrations, or polycations are present. Thus, cleavage with subtilisin affects assembly in a manner similar to the addition of these agents. It appears that binding of positively charged substances may act by neutralizing the charge on the highly acidic C‐terminal regions of the α‐ and β‐subunits, while cleavage with subtilisin produces the same effect by removing these peptides. Undigested and subtilisin‐digested tubulin form sheets of protofilaments in the presence of Zn2+, which indicates that the binding sites for the 2–3 Zn2+ions necessary to induce sheet formation do not reside in the C‐terminal r