Actin rich cortex

The cortical region of many cells is enriched in actin and associated actin-binding proteins, which function in motility, cell shape maintenance, and membrane protein distribution in polarized cells. In some cases, discrete structures anchor actin to the membrane, as is the case for intercellular adherens junctions and cell-substrate focal contacts. In certain special cell types, the fundamental blueprint for an adherens junction is taken to a new structural level, serving as scaffolding for cell-type specific complexes, such as the dystrophin-associated protein complex (DPC) in striated muscle. Although for years morphological studies have described a close association with IF with the actin-rich cortex, recent advances in methods to study protein-protein interactions have provided new insight into the intimate structural and functional relationship between IFs and these membrane domains. 

Although much of the focus has been on the DPC of striated muscle, it is likely that desmin attachments to dense plaques of smooth muscle play critical roles in regulating the transmission of contractile forces in this tissue as well. This is particularly relevant in light of the observed defects in smooth muscle of desmin-deficient mice, in which active force per cross-sectional area was reduced to 40% of controls of smooth muscle tissue (Sjuve et al, 1998). IFAP candidates for serving this linking function are plectin and other components of the actin-rich cortex, including calponin (which also plays a role in the cytoplasm of smooth muscle cell dense bodies see below), and the spectrin/ankyrin complex. 

Dynein seems to be a complementary partner of kInesIn and myosin In trafficking vesicles and chromosomes. Although an emerging theme Is that dynactin couples a membrane cargo to dynein or kinesin, how microtubule-based movements are coordinated with myosin-based movements remains an unresolved question. Several lines of Investigation suggest that dynein could play a central role In coordination. Because the (+) ends of actin filaments and microtubules tend to point toward the cell periphery, dynein Is the only major class of motors that moves toward the cell Interior. The location of cytoplasmic dynein In the actin-rich cortex Is similar to that of myosin and distinct from that of kinesin. Whether dynactin also couples myosin to membrane trafficking Is unknown. 

More recent work suggested that one of four 4.1 proteins—4.1R—may associate with neurofilament proteins in forebrain postsynaptic densities, thus regulating the associated spectrin-rich cortex (Scott et al, 2001). Blot overlay analyses demonstrated that, in addition to spectrin and actin, postsynaptic density polypeptides included NF-L and o-internexin as interacting partners for 4.1R. Collectively, these studies emphasize that common themes are used in different cell types to both strengthen plasma membrane domains enriched in actin and IF polypeptides and to coordinate these sites with cytoplasmic architecture. 

A FIGURE 20-24 Cooperation of myosin and kinesin at the cell cortex. Microtubules approach the actin-rich cell membrane. Consequently, some cargoes are transported to the cell periphery by kinesin motor proteins on microtubules but complete the journey on microfilaments under the power of myosin motor proteins. 

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