Identification of a membrane skeleton in platelets.
Platelets have previously been shown to contain actin filaments that are linked, through actin-binding protein, to the glycoprotein (GP) Ib-IX complex, GP Ia, GP IIa, and an unidentified GP of Mr 250,000 on the plasma membrane. The objective of the present study was to use a morphological approach to examine the distribution of these membrane-bound filaments within platelets. Preliminary experiments showed that the Triton X-100 lysis buffers used previously to solubilize platelets completely disrupt the three-dimensional organization of the cytoskeletons. Conditions were established that minimized these postlysis changes. The cytoskeletons remained as platelet-shaped structures. These structures consisted of a network of long actin filaments and a more amorphous layer that outlined the periphery. When Ca2+ was present, the long actin filaments were lost but the amorphous layer at the periphery remained; conditions were established in which this amorphous layer retained the outline of the platelet from which it originated. Immunocytochemical experiments showed that the GP Ib-IX complex and actin-binding protein were associated with the amorphous layer. Analysis of the amorphous material on SDS-polyacrylamide gels showed that it contained actin, actin-binding protein, and all actin-bound GP Ib-IX. Although actin filaments could not be visualized in thin section, the actin presumably was in a filamentous form because it was solubilized by DNase I and bound phalloidin. These studies show that platelets contain a membrane skeleton and suggest that it is distinct from the network of cytoplasmic actin filaments. This membrane skeleton exists as a submembranous lining that, by analogy to the erythrocyte membrane skeleton, may stabilize the plasma membrane and contribute to determining its shape.
Fox
J E
JE
Department of Pathology, Gladstone Foundation Laboratories for Cardiovascular Disease, University of California, San Francisco 94140-0608.
Boyles
J K
JK
Berndt
M C
MC
Steffen
P K
PK
Anderson
L K
LK
D000199:Actins / Q000032:analysis; D001792:Blood Platelets / Q000648:ultrastructure*; D002462:Cell Membrane / Q000648:ultrastructure; D002498:Centrifugation; D003599:Cytoskeleton / Q000032:analysis / Q000648:ultrastructure*; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007150:Immunohistochemistry; D008840:Microfilament Proteins / Q000032:analysis; D008854:Microscopy, Electron; D010980:Platelet Membrane Glycoproteins / Q000032:analysis
D000199:Actins; D008840:Microfilament Proteins; D010980:Platelet Membrane Glycoproteins