26.6: Microtubule Associated Motor Proteins
Eukaryotic cells have different motor proteins for transporting various cargo within the cell. These motor proteins differ based on the filament they associate with, the direction they move within the cell, and the type of cargo they transport. Motor proteins that associate with microtubules are known as microtubule-associated motor proteins. There are two families of microtubule-associated motor proteins —Kinesins and Dyneins. Both these proteins assist in the transport of cellular cargos within the cell by hydrolyzing ATP molecules. The wide spectrum of cellular cargos transported includes organelles, vesicles, protein complexes, chromosomes, RNA-protein complexes, etc.
Kinesins
Kinesins are found in all eukaryotes. The human genome has forty-five genes encoding kinesin proteins, Arabidopsis thaliana has sixty, while the budding yeast Saccharomyces cerevisiae has six. Kinesin-1 (conventional kinesin) was the first molecular motor protein discovered in a squid neuron. It is involved in transporting cellular cargo via the microtubules. These motor proteins have a highly conserved motor domain of ∼340 amino-acid residues at its N-terminal, with some exceptions. The kinesin superfamily is broadly divided into 14 families with distinct members based on their conserved motor domains. These proteins are plus end-directed, i.e., the transport of the organelles and vesicles occurs from the center of the cell towards the cell periphery, showing the centrifugal movement of cellular cargos. This motor protein superfamily members are also involved in microtubule destabilization, DNA repair, transcription regulation, mitotic spindle assembly, and cell signaling during cell growth regulation.
Dyneins
Dynein is a family of minus-end directed motor proteins, i.e., it transports cellular cargos from the cell periphery towards the center of the cell, showing centripetal movement. They were initially identified from the core of eukaryotic cilia and flagella. The core, also termed axoneme, is where the dyneins make the microtubules slide, resulting in the characteristic whip-like movement of cilia and flagella. These motor proteins are widely present in eukaryotes except for most flowering plants, which lack dynein motor proteins.
Dyneins are classified as axonemal and cytoplasmic based on their location and functions. Cytoplasmic dynein is responsible for intraflagellar transport, while axonemal dynein is involved in the locomotory function of cilia and flagella. During interphase, these motor proteins transport vesicles, organelles, proteins, and mRNA particles, while in dividing cells, these proteins are responsible for spindle assembly.
