Background: Mammalian megakaryocytes release blood platelets through a remarkable process of cytoplasmic fragmentation and de novo assembly of a marginal microtubule band. Cell-specific components of this process include the divergent β-tubulin isoform β1 that is expressed exclusively, and is the predominant isoform, in platelets and megakaryocytes. The functional significance of this restricted expression, and indeed of the surprisingly large repertoire of metazoan tubulin genes, is unclear. Fungal tubulin isoforms appear to be functionally redundant, and all mammalian β-tubulins can assemble in a variety of microtubules, whereas selected fly and worm β-tubulins are essential in spermatogenesis and neurogenesis. To address the essential role of β1-tubulin in its natural context, we generated mice with targeted gene disruption.

Results: β1-tubulin^−/− mice have thrombocytopenia resulting from a defect in generating proplatelets, the immediate precursors of blood platelets. Circulating platelets lack the characteristic discoid shape and have defective marginal bands with reduced microtubule coilings. β1-tubulin^−/− mice also have a prolonged bleeding time, and their platelets show an attenuated response to thrombin. Two alternative tubulin isoforms, β2 and β5, are overexpressed, and the total β-tubulin content of β1-tubulin^−/− megakaryocytes is normal. However, these isoforms assemble much less efficiently into platelet microtubules and are thus unable to compensate completely for the absence of β1-tubulin.

Conclusions: This is the first genetic study to address the essential functions of a mammalian tubulin isoform in vivo. The results establish a specialized role for β1-tubulin in platelet synthesis, structure, and function.