Cytoskeletal-matrix proteins

Integrins are a family of transmembrane heterodimeric glycoproteins that are receptors for specific epitopes of extracellular matrix proteins and for other cell-surface molecules (Kramer et al, 1993). Integrins exist as a dimer complex composed of an a-subunit (120-180 kD) noncovalently associated with a /1-subunit (90-110 kD) (Hynes, 1992). At least 8 /1-subunits and 14 -units have been identified and are concentrated at loci, called focal adhesion sites, of close proximity between cells and extracellular matrices on substrates (Hynes, 1992). Focal adhesion sites are points of aggregation of, and are physically associated with, intracellular cytoskeletal molecules that control, direct, and modulate cell function in response to extracellular signals (Schwartz, 1992). 

The effector caspases (caspase-3, caspase-6, caspase-7) are responsible for the morphological and biochemical changes that mark apoptosis. Activation of the effector caspases occurs via cleavage of the proform by activated initiator caspases and often marks the point of no return for cell death. Substrates for effector caspases include the caspases themselves (autoactivation), cytoskeletal components (i.e., actin, fodrin, and cytokeratins), poly (ADP-ribose) polymerase (PARP), and nuclear matrix proteins like Lamin B. Detection of caspase-3 expression by immunohistochemistry has been studied extensively due to its apical position in the effector caspase cascade (7-9,11-16). As with the initiator caspases, it is important to determine which form of the enzyme is recognized by the spe-... 

Neuromuscular transmission is initiated when an action potential travelling down the axon of a motor neurone arrives at the nerve terminal. Vesicles containing the neuromuscular transmitter, acetylcholine, are present in rows which line up either side of an "active zone". Active zones are transmitter release sites. The vesicles arrive at the active zones guided by the cytoskeletal matrix which is made up of microtubules, the contractile protein actin and the smooth endoplasmic reticulum. Attachment to the cytoskeleton is mediated by various proteins including s mapsin... 

Thus far, microtubules and actin filaments and their associated proteins have been discussed to advantage as independent cytoskeletal components. In actual fact, all of the components of the cytoskeleton (including intermediate filaments) are precisely integrated with one another (Langford, 1995), as well as with various cytoplasmic organelles, the nuclear membrane, the plasma membrane, and the extracellular matrix. In its totality the cytoskeleton subserves many coordinated and regulated functions in the cell ... 

SH3 domains are used extensively by cytoskeletal and signaling proteins to mediate protein-protein interactions, and they do so through a proline-rich motif. This has a consensus sequence P-X-X-P. Another motif—the WW domain—also facilitates protein-protein interactions, and it too is based on proline. Its consensus sequence is P-P-X-Y (a Type I WW repeat as identified in the extracellular matrix receptor /3-dystroglycan) and this interacts with a WW domain in dystrophin or utrophin (Ilsley et al, 2002 Winder, 2001). These two motifs are interesting, not only because they are short and proline-rich, but because they are able to impose considerable specificity of interaction on the proteins involved. 

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