It has been shown that a Triton X-100-insoluble
protein matrix can be isolated from the plasma membranes of P815
tumor cells and murine lymphoid cells (Mescher, M. F., M. J. L. Jose and S. P. Balk, 1981, Nature (Lond.), 289:139-144). The properties of the matrix suggested that this set of
proteins might form a membrane skeletal structure, stable in the absence of the
lipid bilayer. Since purification of plasma membrane results in yields of only 20 to 40%, it was not clear whether the matrix was associated with the entire plasma membrane. To determine if a
detergent-insoluble structure was present over the entire cell periphery and stable in the absence of the membrane bilayer or cytoskeletal components, we have examined extraction of whole cells with
Triton X-100. Using the same conditions as those used for isolation of the matrix from membranes, we found that extraction of intact cells resulted in structures consisting of a continuous layer of
protein at the periphery, a largely empty cytoplasmic space, and a nuclear remnant. Little or no
lipid bilayer structure was evident in association with the peripheral layer, and no filamentous cytoskeletal structures could be seen in the cytoplasmic space by thin-section electron microscopy. Analysis of these Triton shells showed them to retain approximately 15% of the total cell
protein, most of which was accounted for by low molecular weight
nuclear proteins.
5'-Nucleotidase, a cell surface
enzyme that remains associated with the plasma membrane matrix, was quantitatively recovered with the shells. Included among the
polypeptides present in the shells was a set with mobilities identical to those of the set that makes up the plasma membrane matrix. The
polypeptide composition of the shells further confirmed that
cytoskeletal proteins were present to a very low extent, if at all, after the extraction. The results demonstrate that a
detergent-insoluble
protein matrix associated with the periphery of these cells forms a continuous, intact macrostructure whose stability is independent of the membrane bilayer or filamentous cytoskeletal elements, and thus has the properties of a membrane skeletal structure. Although not yet directly demonstrated, the results also strongly suggest that this peripheral layer is composed of the previously described set of plasma membrane matrix
proteins. This article discusses possible roles for this proposed membrane skeletal structure in stabilizing the membrane bilayer and affecting the dynamics of other
membrane proteins.