Figure 5.
Deletions of the N or C terminus of Op18 result in loss of catastrophing promotion or tubulin sequestration, respectively. (A and B) Microtubule plus end elongation rate (A) and catastrophe frequency (B) at pH 6.8 for samples containing 11 μM tubulin and 1.7 μM Op18F or truncated proteins. For comparison, values for tubulin assembled at 7, 9, and 11 μM are shown (open symbols). The data for samples containing Op18F or truncated proteins are offset along the x-axis for clarity. (A) The full-length, epitope-tagged Op18F (closed circles) and the N-terminal truncation (Δ5–25; closed diamonds) slow microtubule elongation, whereas the C-terminal truncation (Δ100–147; closed squares) does not. As shown in B, each of these proteins stimulates catastrophes at pH 6.8. Catastrophe stimulation by the C-terminal truncation (Δ100–147; closed squares) is lower than that by the other proteins but is still 1–5-fold greater than tubulin alone. (C and D) Microtubule plus end elongation rates (C) and catastrophe frequency (D) at pH 7.5. All samples contained 11 μM tubulin. Op18F or Op18F-Δ100–147 was added at 1.7 μM, whereas the N-terminal truncation (Op18F-Δ5–25) was added at a higher concentration, 2.7 μM. As shown in C, Op18F and the truncated proteins did not slow microtubule elongation rate, consistent with the results shown in Figure 2 for wild-type Op18. (D) Both Op18F and the C-terminal truncated protein (Δ100–147) stimulate plus end catastrophes. In contrast, the N-terminal truncated protein (Δ5–25) was inactive, even at a higher concentration.