Beyond cell-cell adhesion: Alpha-catenin organization and function in cellular dynamics and regulation of the actin cytoskeleton

Regulation of cell-cell adhesion is important for numerous biological processes including cell migration, proliferation, and differentiation. One complex involved in cellular adhesion, the adherens junction, is formed upon homophilic binding between cadherins from opposing cells. This calcium-dependent extracellular adhesion is transduced intracellularly through binding to beta-catenin, which in turn binds alpha-catenin. Alpha-catenin has also been shown to bind actin filaments in vitro, which has led to the widely accepted view that cadherins are directly linked to the actin cytoskeleton through alpha-catenin. However, recent data from our lab demonstrated that alpha-catenin cannot bind simultaneously to both the cadherin-beta-catenin complex and actin filaments in vitro. This mutually exclusive binding of alpha-catenin to actin and beta-catenin can be explained by alpha-catenin binding preferences: alpha-catenin monomer preferentially binds the cadherin-beta-catenin complex while the homodimer preferentially binds actin filaments. These data have led to a new model of adherens junction formation in which the allosteric regulation of alpha-catenin reorganizes actin at nascent sites of cell-cell contact to strengthen cellular adhesion.;While the in vitro data are compelling, the existence of a monomer-dimer transition and its effects on actin have not been demonstrated in vivo. Therefore, my aim was to identify the molecular states of alpha-catenin and to test the model by perturbing the endogenous localization of alpha-catenin in vivo. Based upon biochemical data, alpha-catenin in cells is predicted to reside in four functionally distinct pools: (1) cadherin-bound monomer; (2) cytoplasmic monomer; (3) cytoplasmic homodimer; and (4) cytoplasmic homodimer associated with actin. To identify these pools I fractionated MDCK cells into membrane and cytosol, and determined the distribution, binding partners and oligomer state of alpha-catenin. To determine what role these pools of alpha-catenin had on cellular dynamics, I sequestered the cytosolic pool of alpha-catenin to the plasma membrane or to mitochondria, thereby increasing or decreasing, respectively, the potential for alpha-catenin dimerization.;These data confirm that the dynamic relationship between membrane-bound and cytosolic pools of alpha-catenin plays an important role in regulating actin dynamics and membrane activity. In addition, alpha-catenin appears to be a general regulator of cell dynamics at the single cell level, in addition to its role in cell-cell adhesion in a multicellular context.