Vinculin

In mammalian cells, vinculin is a membrane-cytoskeletal protein in focal adhesion plaques that is involved in linkage of integrin adhesion molecules to the actin cytoskeleton. Its sequence is 20%-30% similar to α-catenin, which serves a similar function.

Binding alternately to talin or α-actinin, vinculin's shape and, as a consequence, its binding properties are changed. The vinculin gene occurs as a single copy and what appears to be no close relative to take over functions in its absence. Its splice variant metavinculin (see below) also needs vinculin to heterodimerize and work in a dependent fashion.

Structure
Vinculin is a 117-kDa cytoskeletal protein with 1066 amino acids. The protein contains an acidic N-terminal domain and a basic C-terminal domain separated by a proline-rich middle segment. Vinculin consists of a globular head domain that contains binding sites for talin and α-actinin as well as a tyrosine phosphorylation site, while the tail region contains binding sites for F-actin, paxillin, and lipids (Goldman et al 2001).

Conformation
The recent discovery of the 3D structure sheds light on how this protein tailors its shape to perform a variety of functions. For example, vinculin is able to control the cell’s motility by simply altering its shape from active to inactive. When in its ‘inactive’ state, vinculin’s conformation is characterized by the interaction between its head and tail domains. And, when transforming to the ‘active’ form, such as when talin triggers binding, the intramolecular interaction between the tail and head is severed. In other words, when talin’s binding sites (VBS) of α-helices bind to a helical bundle structure in vinculin’s head domain, the ‘helical bundle conversion’ is initiated, which leads to the reorganization of the α-helices (α1- α-4), resulting in an entirely new five-helical bundle structure. This function also extends to cancer cells, and regulating their movement and proliferation of cancer to other parts of the body.

Mechanism and Function
Background

Cell spreading and movement occur though the process of binding of cell surface integrin receptors to extracellular matrix adhesion molecules. Vinculin is associated with focal adhesion and adherens junctions, which are complexes that nucleates actin filaments and crosslinkers between the external medium, plasma membrane, and actin cytoskeleton (Xu et al 1998). The complex at the focal adhesions consists of several proteins such as vinculin, α-actin, paxillin, and talin, at the intracellular face of the plasma membrane.

In more specific terms, the amino-terminal of vinculin binds to talin, which, in turn, binds to β-integrins, and the carboxy-terminal binds to actin, phospholipids, and paxillin-forming homodimers. The binding of vinculin to talin and actin is regulated by polyphosphoinositides and inhibited by acidic phospholipids. The complex then serves to anchor actin filaments to the membrane (Ezzell et al 1997).

The loss of vinculin impacts a variety of cell functions; it disrupts the formation of the complex, and prevents cell adhesion and spreading. The absence of the protein demonstrates a decrease in spreading of cells, accompanied by reduced stress fiber formation, formation of fewer focal adhesions, and inhibition of lamellipodia extension (Goldman et al 2001). It was discovered that cells that are deficient in vinculin have growth cones that advance more slowly, as well as filopodia and lamellipoida that were less stable then the wild-type. Based on research, it has been postulated that the lack of vinculin may decrease cell adhesion by inhibiting focal adhesion assembly and preventing actin polymerization. On the other hand, overexpression of vinculin may restore adhesion and spreading by promoting recruitment of cyotskletal proteins to the focal adhesion complex at the site of integrin binding (Ezzell et al 1997). Vinculin's ability to interact with integrins to the cytoskeleton at the focal adhesion appears to be critical for control of cytoskeletal mechanics, cell spreading, and lamellipodia formation. Thus, vinculin appears to play a key role in shape control based on its ability to modulate focal adhesion structure and function.

Splice variant: Metavinculin
Smooth muscles and skeletal muscles (and probably to a lower extent in cardiac muscle) in their well-differentiated (contractile) state co-express (along with vinculin) a splice variant carrying an extra exon in the 3' coding region, thus encoding a longer isoform meta-vinculin (meta VCL) of ~150KD molecular weight — a protein whose existence has been known since 1980s. Translation of the extra exon causes a 68- to 79-amino acid acid-rich insert between helices I and II within the C-terminal tail domain. Mutations within the insert region correlate with hereditary idiopathic dilated cardiomyopathy

Length of the insert in metavinculin is 68AA in mammals 79 in frog. Strasser et al compared metavinculin sequences from pig, man, chicken, and frog, and found the insert to be bipartite: the first part variable and the second highly conserved.

Both vinculin isoforms co-localize in muscular adhesive structures, such as dense plaques in smooth muscles, intercalated discs in cardiomyocytes, and costameres in skeletal muscles. Metavinculin tail domain has a lower affinity for the head as compared with the vinculin tail. In case of metavinculin, unfurling of the C-terminal hydrophobic hairpin loop of tail domain is impaired by the negative charges of the 68-amino acid insert, thus requiring phospholipid-activated regular isoform of vinculin to fully activate the metavinculin molecule.