Here we show that Pin1, a peptidyl-prolyl cis/trans isomerase which catalyzes the isomerization of phosphorylated Ser/Thr-Pro, is a regulatory molecule of thrombopoiesis. We found that mice lacking the Pin1 gene (Pin1(-/-) mice) formed more megakaryocytes (MKs) than wild type mice (WT mice), and that the proplatelet formation of MKs was poorer in Pin1(-/-) mice than WT mice. Treatment of Meg-01 cells, a megakaryoblastic floating cell line, with shRNA against Pin1 suppressed the proplatelet formation. Expression of tau, a microtubule associated protein was induced in MKs during proplatelet formation. Pin1 bound tau and promoted microtubule polymerization. Our results show that Pin1 serves as a positive regulatory molecule of proplatelet formation of MKs by enhancing the function of phosphorylated tau. (C) 2017 Elsevier Inc. All rights reserved.