Most important cargos, including crucial proteins and organelles, are manufactured at regions of the cell that are microns from their final destinations. On this scale, distance and time constraints make relying on passive diffusion to deliver cargos impossible, and in order to sustain itself, the cell must invoke active transport by a specialized class of enzymes known as motor proteins. This group of proteins uses the chemical energy from the hydrolysis of the phosphate bonds in adenosine triphosphate (ATP) to undergo global conformational changes capable of generating piconewtonscale forces to distribute essential intracellular cargos, including: organelles, such as mitochondria, vesicles, receptors, and transmembrane proteins.
For example, the transport of subcellular commodities is particularly important to neuronal physiology. Neurons have cell bodies that are around 6 - 100 μm in diameter, but have axonal projections that can extend up to a meter or more. The unique structure of neurons has aided the identification of kinesin-1 and cytoplasmic dynein as the primary microtubule-dependent motor proteins responsible for the long-distance, fast trafficking of materials in the neuron. The microtubules that these motors interact with are long protein polymers and motors are classified by their directionality with respect to microtubule polymerization. Microtubules tend to assemble faster from a point of nucleation, including organizing centers such as the centrosome, to the cell boundary, termed the 'plus-end' direction. While the terms 'minus-end' refers to the slower growing portion of the microtubule proximal to the nucleus. In this manner, kinesin is considered a plus-end directed motor and dynein is termed a minus-end directed motor.
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