The cadherin protein family are common cell-adhesion molecules (CAMs) that mediate cell-cell contacts at anchoring junctions (e.g. adherens junctions, desmosomes) and at prominent sites of cell-cell communication (e.g. neuronal synapses). There are over 100 different cadherin family members that are grouped into at least 6 subfamilies, including type I classical cadherins, type II atypical cadherins and desmosomal cadherins. All cadherins share a common architecture in their extracellular domain that comprises cadherin repeats, with classical cadherins containing five of these repeats. Subtle differences between cadherins impart each type with specificity for particular tissue and cell types. Cadherins use a common set of adaptor molecules and pathways to facilitate cell adhesion and communication, however the strength of adhesion varies with the type of cadherin present.
The structural link between the actin cytoskeleton and cadherins in adherens junctions is comprised of direct interactions in the following order: cadherin tails bind beta-catenin, beta-catenin binds the VH1 domain of alpha-catenin, and the VH3 domain of alpha-catenin binds actin. Although alpha-catenin is known to be essential in this link, recent biochemical and in vitro studies by the Weis and Nelson group has shown that a direct link between the cytoskeleton and alpha-catenin is unlikely to occur in vivo, and instead, is likely mediated by additional adaptor proteins.
Actomyosin refers to the actin-myosin complex that forms within the cytoskeleton. Actomyosin is inherently contractile, with the myosin motor protein able to pull on actin filaments. This property gives rise to contractile fibers that form the basis of skeletal muscle, and even in non-muscle cells, enable cell motility and force generation at the sub-cellular level. Actin filament networks, both within filopodia and lamellipodia, are highly dynamic structures. A number of cellular processes such as filopodial retraction and lamellipodial/lamellal contractions, rely on the rearward movement of the whole filament network or large filament bundles. The retrograde motion of actin treadmilling may play a minor role in aiding these processes. One class of proteins that has been implicated in the translocation of F-actin is the myosin motor protein family. Each member of the myosin family possesses unique structural and functional properties, such as their step size, that determines their ability to engage in F-actin translocation. Myosin II specifically, has been associated with F-actin retraction in several cell types including neurons, fibroblasts and keratocytes.