Axonal transport of tubulin and actin.

Axonal transport is responsible for supplying the axonal processes with proteins that are synthesized in the cell body. Among the proteins that are moved by this mechanism are tubulin and actin, two major components of the cytoskeleton. Observation of the movement of metabolically labeled tubulin and actin in-vivo has demonstrated that tubulin and actin transport are reduced in various diseases and with age, but transport is increased during axonal growth and regeneration. These metabolic studies have also raised questions about the underlying mechanisms of slow axonal transport such as: what is the polymerization state of tubulin and actin during transport, what motors and tracks are responsible for their movement down the axon, and how are the transport motors coupled to tubulin and actin during transport? Since experiments using metabolically labeled tubulin and actin have not effectively addressed these questions, a variety of new in-vitro fluorescent microscopy techniques have been devised to investigate these questions. These fluorescent microscopy experiments have suggested that tubulin can be transported in the unpolymerized soluble state and that such transport of soluble tubulin relies on the presence of formed microtubule tracks. It is not yet known what motor or motors are responsible for tubulin or actin transport in axons or how such a motor(s) might be coupled to such an abundant soluble cargo.