Megakaryocytes Package and Deliver Golgi-Associated Glycosyltransferases into Platelets and to Platelet Surfaces Using Dense Granules.

We recently presented evidence for the presence of a β4 galactosyl transferase (β4GalT) on the surface of platelets that catalyzes the coupling of galactose in a β1,4 linkage to exposed N-acetylglucosamine residues on the N-linked glycans of GP1ba and that this reaction improved the circulation of chilled murine platelets. Since an externally-disposed glycosyltransferase activity contradicts the dogma that such enzymes reside solely within the Golgi apparatus, we verified it by demonstrating that intact platelets glycosylate 2.8 μm beads coated with the acceptor substrate GlcNAc-β-R. We then explored the origin of this enzyme in megakaryocytes and its mechanism of delivery to platelets. We report that Golgi marker GM130 as well as a YFP tagged Golgi marker construct based on the polypeptide transferase GalNAc-T2 expressed in cultured mouse megakaryocytes transports from the cell body of to nascent platelets in discrete packets. In immature megakaryocytes, the Golgi organizes, as expected, into perinuclear arrays. However, once proplatelet extension begins, the perinuclear apparatus disassembles, and Golgi-associated proteins transport through the proplatelets in vesicular/granular structures. Labeling with granule markers reveals correspondence of Golgi markers only with dense granules. These findings suggest that Golgi membrane becomes packaged into dense granules, which deliver Golgi-associated glycosyltransferases into the nascent platelets and from there to the platelet surface.