The C-terminal CNBr peptide "α1CT" is longer than, but includes the structural element "α1CTT", which is the disordered C-Terminal Tail peptide that extends from the surface of the tubulin dimer and from the microtubule outer surface. The glutamic acid (E) residue labeled in red and with an asterisk (E406) is a site of posttranslational modification by addition of one E (or more) to the γ-carboxyl of the main chain E residue.