A role for Sec1/Munc18 proteins in platelet exocytosis.
A critical aspect of haemostasis is the release of clot-forming components from the three intra-platelet stores: dense-core granules, alpha granules and lysosomes. Exocytosis from these granules is mediated by soluble proteins [N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs)] and integral membrane proteins [vesicle and target SNAP receptors (v- and t-SNAREs)]. Three Sec1/Munc18 proteins (SM proteins) are present in platelets (Munc18a, Munc18b and Munc18c) and they bind to and potentially regulate specific syntaxin t-SNAREs. In resting platelets, these SM proteins associate with granules and open canalicular system membranes predominantly but not with the plasma membrane. Munc18a binds to syntaxin 2 alone and does not associate with other members of the core SNARE complex. Munc18b associates with a larger complex that contains synaptosome-associated protein of 23 kDa (SNAP-23) and cellubrevin/vesicle-associated membrane protein 3. Munc18c associates with both syntaxins 2 and 4, with synaptosome-associated protein of 23 kDa (SNAP-23) and with a v-SNARE. On stimulation, most of the platelet SM proteins are still found in membrane fractions. Phosphorylation of each Munc18 increases in thrombin-treated cells and phosphorylated Munc18c remains associated with syntaxins 2 and 4, but its affinity for the SNAREs appears to be reduced. To determine the functional role of the platelet SM proteins, we examined the effects of Munc18-based peptides (Munc18a peptide 3 and Munc18c peptide 3). Addition of the peptides to permeabilized platelets inhibits secretion from all three platelet granules. These peptides also inhibit agonist-induced aggregation in saponin-permeabilized platelets. These studies demonstrate a clear role for SM proteins in platelet exocytosis and aggregation and suggest a dominant role for Munc18c in all three granule-release events.
Schraw
Todd D
TD
Department of Molecular and Cellular Biochemistry, College of Medicine, University of Kentucky, 800 Rose Street, Lexington, KY 40536, USA.
Lemons
Paula P
PP
Dean
William L
WL
Whiteheart
Sidney W
SW
D000595:Amino Acid Sequence; D001483:Base Sequence; D001792:Blood Platelets / Q000502:physiology* / Q000648:ultrastructure; D003001:Cloning, Molecular; D003594:Cytoplasmic Granules / Q000502:physiology / Q000648:ultrastructure; D017931:DNA Primers; D018076:DNA, Complementary / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D005089:Exocytosis / Q000502:physiology*; D015723:Gene Library; D006801:Humans; D008247:Lysosomes / Q000502:physiology / Q000648:ultrastructure; D008565:Membrane Proteins / Q000737:chemistry; D008969:Molecular Sequence Data; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D010446:Peptide Fragments / Q000737:chemistry; D016133:Polymerase Chain Reaction; D011994:Recombinant Proteins / Q000097:blood / Q000737:chemistry; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D051040:Vesicle-Associated Membrane Protein 3; D033921:Vesicular Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*
D017931:DNA Primers; D018076:DNA, Complementary; D008565:Membrane Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; C497937:STXBP1 protein, human; C497940:STXBP2 protein, human; D051040:Vesicle-Associated Membrane Protein 3; D033921:Vesicular Transport Proteins