PLATELET GLYCOPROTEIN IA-ASTERISK IS THE PROCESSED FORM OF MULTIMERINISOLATION AND DETERMINATION OF N-TERMINAL SEQUENCES OF STORED AND RELEASED FORMS

Glycoprotein Ia (GPIa*), a very high molecular mass, platelet alpha-g ranule protein consisting of 167 kDa subunits disulphide-linked in a m ultimeric structure, was first described by Bienz and Clemetson in 198 9 (J. Biol. Chem. 264, 507-514). In 1991 Hayward et al. (J. Biol. Chem . 266, 7114-7120) independently identified a platelet protein with mul timeric structure. Despite strong similarities to GPIa they concluded that it was a novel multimeric protein and named it first p-155 and l ater, multimerin. Multimerin has also been found in endothelial cells and has been cloned recently from an endothelial cell cDNA library, Th is has made it possible for us to clarify the relationship between GPI a and multimerin. GPIa* was isolated from platelet releasate and the N-terminal sequence of 167 kDa and 155 kDa subunit species were determ ined. The N-terminal 15 amino acids of GPIa were identical to the ded uced amino acids 184-198 of endothelial multimerin. The N-terminal seq uence of the 155 kDa protein was identical to the deduced amino acids 318-326 of multimerin. Thus, platelet GPIa (167 kDa) is the main proc essed form of multimerin stored in platelet alpha-granules. The GPIa/ processed multimerin (167 kDa) still contains an RGDS sequence near it s N-terminus as well as an EGF domain which may be involved in binding to the platelet surface after release. This sequence and domain are c leaved off in the p-155 form, described earlier as platelet multimerin , which is probably formed after release from alpha-granules.