Bovine brain tubulin does not contain α-tubulin with C-terminal taurine, and taurine modification is not produced with enzymatic treatments. All panels show the m/z region corresponding to bovine α1CT collected by MALDI-TOF MS. The arrow in all panels shows the position of the expected mass of taurinated CT, i.e., α1CT-Y + taurine, if it occurred. (A) Bovine brain tubulin (BBT) sample, untreated. (B) BBT treatment with CPA to remove tyrosine as indicated by the loss of signal at m/z 2861. (C) Treatment of BBT with CPA followed by TTL in the presence of tyrosine. Restoration of the full-length peptide at m/z 2861 is observed. (D) Incubation of BBT is the same as (C), but with taurine rather than Tyr and does not produce taurinated CTT, i.e., α1CT-Y + taurine (expected average m/z 2805).