Kinesins are a class of protein complexes known as molecular motors, found in eukaryotic cells. They function by converting chemical energy into mechanical work, enabling movement within the cell. Kinesins navigate along cellular tracks called microtubules, which are components of the cell’s internal scaffolding system. This movement allows kinesins to transport various cellular materials to their correct destinations, playing a fundamental role in maintaining cellular organization and function.
Understanding Kinesins
A typical kinesin molecule is a protein dimer, consisting of two identical heavy chains and two light chains. The heavy chain includes a globular “head” at one end, which serves as the motor domain. This head region possesses two distinct binding sites: one for attaching to the microtubule track and another for binding to adenosine triphosphate (ATP), the cell’s energy currency.
Connected to the motor heads is a flexible “neck linker,” leading into a long, central “stalk” region. The stalks of the two heavy chains intertwine to form a coiled-coil structure. At the opposite end of the stalk is a “tail” domain, where the kinesin binds to its cargo. Kinesins are part of a larger superfamily of proteins, with over 40 different kinesin proteins identified in mammals, organized into at least 14 families.
The Mechanics of Kinesin Movement
Kinesins move along microtubules in a process often described as “walking,” resembling a hand-over-hand motion. This movement is powered by the hydrolysis of ATP. One head of the kinesin binds firmly to the microtubule while the other head detaches and swings forward.
The cycle begins with ATP binding to the leading head, which causes a conformational change in its neck linker. This change “zips” the neck linker onto the motor head, pulling the trailing head forward. The trailing head then binds to a new site on the microtubule, 8 nanometers (nm) away from its previous position. The newly bound head releases a phosphate molecule, a byproduct of ATP hydrolysis, and becomes the new leading head, while the other head detaches and prepares for its next step. This coordinated, cyclical binding and unbinding, fueled by ATP, allows the kinesin to move continuously and unidirectionally along the microtubule, towards its plus end, at speeds of up to 100 steps per second.
Kinesins’ Essential Cellular Functions
Kinesins act as the cell’s internal transport system. They move various components throughout the cytoplasm, including membrane-bound organelles like mitochondria and lysosomes, as well as vesicles containing proteins, hormones, or neurotransmitters. This ensures these materials reach their destinations. This transport is directed towards the cell’s periphery, known as anterograde transport.
Kinesins are also involved in cell division, specifically during mitosis and meiosis. They contribute to the assembly of the spindle apparatus, which is responsible for separating chromosomes. Kinesin family members, such as Kinesin-5 proteins, slide microtubules apart within the spindle, while Kinesin-13 family members help depolymerize microtubules. Both actions contribute to chromosome segregation to daughter cells.
In neurons, kinesins are important for axonal transport, moving materials along axons from the cell body to nerve endings. This transport ensures that components, such as synaptic vesicle precursors, mitochondria, and signaling molecules, are delivered to the synapses, which can be over a meter away in some neurons. This anterograde transport supports neuronal development, connectivity, and the establishment of functional synaptic connections. Beyond transport and division, kinesins also contribute to maintaining cell shape and the positioning of organelles, supporting the structural integrity and organization of the cell.