Figure 10–35 Three domains in the plasma membrane of a guinea pig sperm. (A) A drawing of a guinea pig sperm. (B–D) In the three pairs of micrographs, phase-contrast micrographs are on the left, and the same cell is shown with cell-surface immunofluorescence staining on the right. Different monoclonal antibodies selectively label cell-surface molecules on (B) the anterior head, (c) the posterior head, and (D) the tail. (Micrographs courtesy of Selena carroll and Diana Myles.)
Figure 10–36 Four ways of restricting the lateral mobility of specific plasma membrane proteins. (A) the proteins can self-assemble into large aggregates (as seen for bacteriorhodopsin in the purple membrane of Halobacterium salinarum); they can be tethered by interactions with assemblies of macromolecules (B) outside or (c) inside the cell; or (D) they can interact with proteins on the surface of another cell.
junctional complex spectrin dimer actin ankyrin band 3 glycophorin band 4.1 100 nm ˜ chain H2N HOOC ° chain fexible link between domains 106-amino-acid-long repeating domain NH2spectrin spectrin tetramer plasma membrane
An analogous but much more elaborate and highly dynamic cytoskeletal network exists beneath the plasma membrane of most other cells in our body. This network, which constitutes the cortex of the cell, is rich in actin filaments, which are attached to the plasma membrane in numerous ways. The dynamic remodeling of the cortical actin network provides a driving force for many essential cell functions, including cell movement, endocytosis, and the formation of transient, mobile plasma membrane structures such as filopodia and lamellopodia band 4.1
Figure 10–38 The spectrin-based cytoskeleton on the cytosolic side of the human red blood cell plasma membrane. (A) the arrangement shown in the drawing has been deduced mainly from studies on the interactions of purified proteins in vitro. Spectrin heterodimers (enlarged in the drawing on the right) are linked together into a netlike meshwork by “junctional complexes” (enlarged in the drawing on the left). each spectrin heterodimer consists of two antiparallel, loosely intertwined, flexible polypeptide chains called α and β. the two spectrin chains are attached noncovalently to each other at multiple points, including at both ends. Both the α and β chains are composed largely of repeating domains. two spectrin heterodimers join end-to-end to form tetramers.
the junctional complexes are composed of short actin filaments (containing 13 actin monomers) and these proteins—band 4.1, adducin, and a tropomyosin molecule that probably determines the length of the actin filaments. the cytoskeleton is linked to the membrane through two transmembrane proteins—a multipass protein called band 3 and a single-pass protein called glycophorin. the spectrin tetramers bind to some band 3 proteins via ankyrin molecules, and to glycophorin and band 3 (not shown) via band 4.1 proteins.
(B) the electron micrograph shows the cytoskeleton on the cytosolic side of a red blood cell membrane after fixation and negative staining. the spectrin meshwork has been purposely stretched out to allow the details of its structure to be seen. In a normal cell, the meshwork shown would be much more crowded and occupy only about one-tenth of this area. (B, courtesy of t. Byers and D. Branton, Proc. Natl Acad. Sci. USA 82:6153–6157, 1985. With permission from the national Academy of Sciences.)
Figure 10–37 a scanning electron micrograph of human red blood cells.