Localization of rhodamine-tubulin in platelets. (A) Mouse platelets permeabilized in 0.4% octyl-β-d-glucopyranoside (OG) were incubated with rhodamine-labeled tubulin in 0.1 mM GTP, washed, and then formaldehyde-fixed. Rhodamine-labeled tubulin incorporation at multiple points around the marginal band indicates that free microtubule plus ends exist within the microtubule coil. An average of 7.9 (± 1.9; n = 48) signals of rhodamine-labeled tubulin were observed along the periphery of each platelet. (B) Histogram of fluorescent tubulin signals along the marginal band. The number of signals ranged from 4 to 12, with a median of 8. (C) Fluorescence micrograph of purified mouse platelets that had been permeabilized in 0.5% Triton X-100, incubated in rhodamine-labeled tubulin, and then washed and fixed in formaldehyde. Under these permeabilization conditions, rhodamine-labeled tubulin incorporation occurred at a single site () along the platelet marginal band.