The α-granule proteome: novel proteins in normal and ghost granules in gray platelet syndrome

Deficiencies in granule-bound substances in platelets cause congenital bleeding disorders known as storage pool deficiencies. For disorders such as gray platelet syndrome (GPS), in which thrombocytopenia, enlarged platelets and a paucity of α-granules are observed, only the clinical and histologic states have been defined.

In order to understand the molecular defect in GPS, the α-granule fraction protein composition from a normal individual was compared with that of a GPS patient by mass spectrometry (MS).

Platelet organelles were separated by sucrose gradient ultracentrifugation. Proteins from sedimented fractions were separated by sodium dodecylsulfate polyacrylamide gel electrophoresis, reduced, alkylated, and digested with trypsin. Peptides were analyzed by liquid chromatography-tandem MS. Mascot was used for peptide/protein identification and to determine peptide false-positive rates. MassSieve was used to generate and compare parsimonious lists of proteins.

As compared with control, the normalized peptide hits (NPHs) from soluble, biosynthetic α-granule proteins were markedly decreased or undetected in GPS platelets, whereas the NPHs from soluble, endocytosed α-granule proteins were only moderately affected. The NPHs from membrane-bound α-granule proteins were similar in normal platelets and GPS platelets, although P-selectin and Glut3 were slightly decreased, consistent with immunoelectron microscopy findings in resting platelets. We also identified proteins not previously known to be decreased in GPS, including latent transforming growth factor-β-binding protein 1(LTBP1), a component of the transforming growth factor-β (TGF-β) complex.

Our results support the existence of 'ghost granules' in GPS, point to the basic defect in GPS as failure to incorporate endogenously synthesized megakaryocytic proteins into α-granules, and identify specific new proteins as α-granule inhabitants.