Kinesin motors transport a variety of cargo using a network of intracellular microtubules, as well as being involved in the assembly of bipolar spindles and the depolymerisation of microtubules. Regions outside of the motor domain often determine the cargo specificity of each kinesin. Some kinesin motors associate with light chains, which are polypeptides that form part of the cargo-binding site. Two structural motifs have been identified in KLCs: heptad repeats that bind to KHCs and tetratrico peptide repeats (TPR) that link cargo to kinesin motors. In addition, KLCs can be alternatively spliced to yield diverse C-terminal ends involved in cargo selection. However, some kinesins, such as fungal kinesins, lack light chains.
Kinesins bind their cargo through protein-protein interactions, often with the aid of scaffolding complexes, some of which are involved in signalling pathways. For example, c-Jun N-terminal kinase (JNK)-interacting proteins (JIPs) are scaffolding proteins involved in the JNK/MAP-kinase signalling pathway; kinesins can transport different proteins that associate with JIPs down neuronal axons to stimulate synaptogenesis. Scaffolding proteins are thus able to sort related proteins into a single specific cargo vesicle, as well as being able to interact with the motor that transports these vesicles to their final destination. A kinesin motor may transport multiple, different cargoes, and conversely, any particular cargo may be transported by more than one type of kinesin, which allows a high degree of plasticity to protein trafficking. Some of the types of cargoes that kinesins transport are listed below:
Kinesins can move several membranous structures within a cell, such as mitochondria, endoplasmic reticulum (ER) and post-Golgi vesicles. Kinesins usually bind to receptors or adaptors on the surface of vesicles being transported, which interact with either the KHC or the KLC polypeptides. For example, the Reelin receptor on post-Golgi vesicles binds to KLC, while the kinectin receptor on the ER binds to KHC. These receptors are not specific for kinesin, but usually have other functions as well. Receptor binding often involves the presence of membrane-associated scaffolding proteins and transmembrane proteins. For example, the binding of kinesin I to the Reelin receptor requires the JIP scaffolding protein before the post-Golgi vesicle can be transported down the axon for synaptogenesis. Kinesins do not always require receptors to bind membranes, such as with the kinesin KIK1A, which has a pleckstrin homology domain capable of binding directly to phospholipids.
Through the transport of protein complexes, kinesins play an important role in cell signalling, tissue metamorphosis and development. Protein complexes include the intraflagellar transport (IFT) complexes required for cilia and flagellar formation in a wide variety of organisms. Pre-assembled signalling cascades are often carried by kinesins, whereby the grouping of these signalling cascades ensures the proper trafficking and localisation of signalling molecules.
During meiotic and mitotic cell division, kinesins help ensure the correct segregation of chromosomes by aiding in the assembly and function of the meiotic/mitotic apparatus. Kinesins have four main roles in cell division:
In neurons, a variety of different mRNAs are transported via kinesins into dendrites, where they are then translated. Such RNA trafficking allows a cell to concentrate mRNAs where their translated products are needed, such as near activated synapses for proteins that modify synaptic plasticity. In other regions of the neuron, such as axons that lack ribosomes and endoplasmic reticulum, mRNAs are translated in the cell body and the proteins are then transported by kinesins into the axon in membranous organelles or protein complexes. Other somatic cell types also utilise mRNA trafficking in order to localise proteins within a cells.
Many viruses have learnt to take advantage of intracellular transport systems, using them to move within infected cells. Once a virus enters a cell, it can use the transport system to move to the cell centre where it replicates, the newly synthesized viruses then being transported to the cell periphery for release. Viruses can bind to either the KHC or the KLC: Herpes simplex virus can move within neurons via binding to KHC, while Vaccinia virus is transported within cells after binding to KLC.
Additional information on kinesin and related proteins, as well as on microtubule assembly, can be found at the Kinesin Website: http://www.proweb.org/kinesin//.