Platelets glycosylate exogenous acceptors. Matrix-assisted laser desorption/ionization–time of flight mass spectrometry of glycosylation products in detergent extracts from resting platelets (Platelets), secretion from TRAP-activated platelets (Plt. secretion), Chinese hamster ovary cell lysates after addition of exogenous GalNAc-transferase acceptor TMR-PEG-MUC1 (left), or Gal-transferase acceptor TMR-βGlcNAc (right). No exogenous donor UDP-GalNAc or UDP-Gal was added, with the exception of the top panels, in which 100μM UDP-GalNAc and UDP-Gal were added, respectively (n = 3).