Response of β1-tubulin^—/—platelets to treatment with paclitaxel and properties of β1-tubulin^+/—platelets. (A) Treatment with the MT-stabilizing agent paclitaxel does not change the shape of resting wild-type platelets (+/+, left) or restore discoid morphology in β1-tubulin^—/— platelets (right); original magnification, × 1000. (B) Differential interference contrast (DIC) micrograph of β1-tubulin^+/— platelets in suspension, revealing the discoid resting shape. Inset (original magnification, × 1000) shows antitubulin immunofluorescence of cytocentrifuged β1-tubulin^+/— platelets, which resemble wild-type platelets and lack the kinks and breaks seen in β1-tubulin^—/— platelets. Scale bar = 6 μm. (C) Electron micrograph of resting β1-tubulin^+/— platelet in transverse section, showing about half the number of microtubule (mt) coilings typically seen in the wild type. Scale bar = 0.1 μm. (D) Immunoblot analysis of platelet lysates to show relative proportions of all β-tubulin isotypes in wild-type (+/+) and β1-tubulin^+/— platelets. αtub indicates α-tubulin; GAPDH, glyceraldehyde 3-phosphate dehydrogenase (protein loading control).