Platelet membrane skeleton revealed by quick-freeze deep-etch.

Actin polymerization is an essential component of platelet activation. Since actin appears to polymerize at its membrane-associated end, knowledge of the structural relationship of actin filaments to membrane is an important part of understanding that polymerization process. A membrane-associated actin-containing cytoskeleton has been described in human platelets biochemically and is composed, at least in part, by an association between glycoprotein Ib and the actin-binding protein originally isolated from macrophages. Many other actin-associated proteins with known sub-membranous localization in other systems have been found in platelets, including alpha-actinin, vinculin, and low levels of spectrin and the red cell protein Band 4.1. Because of the density of the platelet cytoplasm, the structure of the membrane-skeleton has not yet been visualized. We have used quick freeze-deep etch techniques to observe the sub-membranous cytoplasm and report visualization of a periodic, submembranous filament system not before seen in the platelet. This filamentous system was more easily observed in thrombin-stimulated platelets, but appeared to be present in resting, discoid cells as well. The filaments could also be readily observed when platelets are lysed after fixation, stained with tannic acid, and embedded for thin-sectioning. This membrane cytoskeleton was composed of 9 nm thick filaments lying 15 nm apart, and 15 nm from the membrane. The filaments appeared to lie in parallel and to encircle the cell. Similar filaments could be seen associated with intracytoplasmic membrane systems in activated cells.