Human platelets contain SNARE proteins and a Sec1p homologue that interacts with syntaxin 4 and is phosphorylated after thrombin activation: implications for platelet secretion.
In response to thrombin and other extracellular activators, platelets secrete molecules from large intracellular vesicles (granules) to initiate thrombosis. Little is known about the molecular machinery responsible for vesicle docking and secretion in platelets and the linkage of that machinery to cell activation. We found that platelet membranes contain a full complement of interacting proteins-VAMP, SNAP-25, and syntaxin 4-that are necessary for vesicle docking and fusion with the plasma membrane. Platelets also contain an uncharacterized homologue of the Sec1p family that appears to regulate vesicle docking through its binding with a cognate syntaxin. This platelet Sec1 protein (PSP) bound to syntaxin 4 and thereby excluded the binding of SNAP-25 with syntaxin 4, an interaction critical to vesicle docking. As predicted by its sequence, PSP was detected predominantly in the platelet cytosol and was phosphorylated in vitro by protein kinase C (PKC), a secretion-linked kinase, incorporating 0.87 +/- 0.11 mol of PO4 per mole of protein. PSP was also specifically phosphorylated in permeabilized platelets after cellular stimulation by phorbol esters or thrombin and this phosphorylation was blocked by the PKC inhibitor Ro-31-8220. Phosphorylation by PKC in vitro inhibited PSP from binding to syntaxin 4. Taken together, these studies indicate that platelets, like neurons and other cells capable of regulated secretion, contain a unique complement of interacting vesicle docking proteins and PSP, a putative regulator of vesicle docking. The PKC-dependent phosphorylation of PSP in activated platelets and its inhibitory effects on syntaxin 4 binding provide a novel functional link that may be important in coupling the processes of cell activation, intracellular signaling, and secretion.
Reed
G L
GL
Cardiovascular Biology Laboratory, Harvard School of Public Health, Cardiac Unit, Massachusetts General Hospital, Boston, MA, USA. reed@cvlab.harvard.edu
Houng
A K
AK
Fitzgerald
M L
ML
D000595:Amino Acid Sequence; D001483:Base Sequence; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D002459:Cell Fusion / Q000502:physiology; D002462:Cell Membrane / Q000502:physiology; D003001:Cloning, Molecular; D003600:Cytosol / Q000378:metabolism; D006801:Humans; D008565:Membrane Proteins / Q000097:blood* / Q000235:genetics; D008969:Molecular Sequence Data; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins / Q000097:blood*; D010766:Phosphorylation; D015539:Platelet Activation; D016133:Polymerase Chain Reaction; D011493:Protein Kinase C / Q000097:blood; D050765:Qa-SNARE Proteins; D011994:Recombinant Proteins / Q000096:biosynthesis; D050600:SNARE Proteins; D050825:Synaptosomal-Associated Protein 25; D013917:Thrombin / Q000494:pharmacology / Q000502:physiology*; D033921:Vesicular Transport Proteins
D008565:Membrane Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D050765:Qa-SNARE Proteins; D011994:Recombinant Proteins; C494208:SNAP25 protein, human; D050600:SNARE Proteins; D050825:Synaptosomal-Associated Protein 25; D033921:Vesicular Transport Proteins; D011493:Protein Kinase C; D013917:Thrombin