The storage defects in grey platelet syndrome and alpha delta-storage pooldeficiency affect alpha-granule factor V and multimerin storage without altering their proteolytic processing

Among proteins stored in alpha -granules, multimerin and factor V share unu sual features: they bind to each other, are proteolysed to unique forms and are stored eccentrically in alpha -granules. These unique features of thei r processing led us to study these proteins in alpha delta storage pool def iciency (alpha delta -SPD) and grey platelet syndrome (GPS, alpha -SPD), tw o conditions known to impair alpha -granule protein storage. Platelet facto r V and multimerin were severely reduced in GPS, whereas they ranged from r educed to normal in alpha delta -SPD. The platelet levels of factor V and m ultimerin in these disorders indicated multimerin deficiency was not predic tive of platelet factor V deficiency, although it reduced the amount of mul timerin associated with platelet factor V, In GPS only, the defect in stori ng proteins was associated with increased multimerin and multimerin-factor V complexes in plasma. Like normal platelets, GPS and alpha delta -SPD plat elets contained factor V mainly in granules. Platelet factor V and multimer in were proteolysed to normal platelet forms in GPS and alpha delta -SPD pl atelets, indicating that these conditions preserve some aspects of normal a lpha -granule protein processing. Although we found factor V can be stored in platelets deficient in multimerin, our data indicate that multimerin sto rage influences the point at which multimerin binds factor V.