Microtubule Associated Motor Proteins

Microtubule-associated motor proteins are classified into two superfamilies: kinesins and dyneins. Kinesins are plus end-directed motors that transport cargo from the cell center toward the periphery. Dyneins are minus end-directed motors that transport cargo from the cell periphery toward the center. Both use ATP hydrolysis to generate movement along microtubules.

Kinesins use a hand-over-hand movement mechanism where their ATP-hydrolyzing globular heads work in tandem to move across microtubules. The globular head binds to the microtubule while the fan-shaped receptor attaches to organelles and vesicles. This coordinated stepping action allows kinesins to transport cargo toward the cell periphery during interphase.

Cytoplasmic dynein consists of heavy, intermediate, light, and light-intermediate chains. The heavy chain contains a globular head responsible for energy generation and a stalk that binds to microtubules. Cytoplasmic dynein associates with dynactin and other large proteins to attach to adaptor-bound cargo before transporting it toward the cell center.

Microtubule-associated motor proteins transport a wide spectrum of cellular cargo including organelles, vesicles, protein complexes, chromosomes, and RNA-protein complexes. During interphase, these motors move vesicles and organelles to specific intracellular locations. In dividing cells, they also participate in spindle assembly and chromosome segregation.

Kinesins are plus end-directed motors that move cargo centrifugally from the cell center toward the periphery. Dyneins are minus end-directed motors that move cargo centripetally from the cell periphery toward the center. This directional difference allows cells to position organelles and vesicles in specific locations for proper cellular function.

Dyneins function in multiple cellular processes beyond cargo transport. Axonemal dyneins in cilia and flagella make microtubules slide, producing the characteristic whip-like movement. Cytoplasmic dyneins are responsible for intraflagellar transport and spindle assembly during cell division, making them essential for both cell motility and mitotic progression.

Beyond cargo transport, kinesins are involved in microtubule destabilization, DNA repair, transcription regulation, and mitotic spindle assembly. They also participate in cell signaling during cell growth regulation. The kinesin superfamily comprises 14 families with distinct members, each specialized for specific cellular functions.