The cytoskeleton and plasma membrane.

The major cytoskeletal elements (microfilaments, microtubules, and 10-nm filaments) are frequently found attached to or near the plasma membrane in arrays which can sometimes be shown experimentally to be related to cell form and movement. Ultrastructural investigations show that attachment is direct, by amorphous electron-dense material, by cell-cell junctions, or by cell-substrate attachment sites, but the chemistry of attachment is poorly understood. The structural and functional polarity of the attached elements has been defined for some microfilaments attached to plasma membrane, but this important parameter has been investigated only slightly for microtubules and not at all for 10-nm filaments attached to membranes. Assemblies of cytoskeletal elements and plasma membrane evidently are structurally stable enough to be observable by electron microscopy and to survive isolation by the usual biochemical techniques, but observations of living cells show that many assemblies of cytoskeleton and plasma membrane undergo rearrangement and interconversion. The biochemical basis and physiological meaning of many of these changes are poorly understood.