Without α–E-catenin (green), dynactin (red) and its associated organelles remain on the microtubule highways (left).

If microtubules are the highways of the cell, then actin filaments are the local roads. Lien et al. suggest that α–E-catenin might sit at the junction between the two and help organelles transit from one to the other—a curious discovery for a protein commonly known to operate at an entirely different type of junction.

α–E-catenin is a major component of adherens junctions, binding via β-catenin to transmembrane protein E-cadherin to hold neighboring cells together. However, α–E-catenin is also suspected to control cell proliferation. Lien et al. set out to discover how the protein performs this alternate function, by searching for new α–E-catenin interaction partners. Much to the authors' surprise they identified dynamitin—a crucial piece of microtubule motor machinery, and not an immediately obvious candidate in proliferation control.

The microtubule motors traffic organelles around the cell, and in cells that lacked α–E-catenin this trafficking was faster. This might be caused by a reduced connection between microtubules and actin; α–E-catenin is a known actin binder, and similarly increased organelle speeds are seen in cells with disrupted actin filaments. Indeed, when the team replaced endogenous α–E-catenin with a version of the protein that could not bind actin (but that still bound dynamitin), organelle speeds matched that of cells that lacked α–E-catenin entirely. Thus, α–E-catenin might slow organelles' passage along microtubules by continually tempting them to take the scenic actin route. RW

Lien, W.-H., et al.
J. Cell Biol.