Summary

α-Granule protein storage is important for producing plateletswith normal haemostatic function.The low to undetectable levelsof several megakaryocyte-synthesized α-granule proteins innormal plasma suggest megakaryocytes are important tosequester these proteins in vivo. α-Granule protein storage invitro has been studied using other cell types, with differencesobserved in how some proteins are processed compared toplatelets. Human megakaryocytes, cultured from cord bloodCD34⁺ cells and grown in serum-free media containing thrombopoietin,were investigated to determine if they could be usedas a model for studying normal α-granule protein processingand storage. ELISA indicated that cultured megakaryocytes con-Correspondencetained the a -granule proteins multimerin, von Willebrand factor,thrombospondin-1, β-thromboglobulin and platelet factor4, but no detectable fibrinogen and factor V. A significant pro-portionof the α-granule protein in megakaryocyte cultureswas contained within the cells (averages: 41 – 71 %), consistentwith storage. Detailed analyses of multimerin and vonWillebrand factor confirmed that α-granule proteins were processedto mature forms and were predominantly located in theα-granules of cultured megakaryocytes.Thrombopoietin-stimulatedcultured megakaryocytes provide a useful model for studyingα-granule protein processing and storage.