All are involved in fast axonal transport, except
Correct Answer: Neurofilaments
Description: Ans. is 'd' ie Neurofilament Intracellular transport is the one of the most important function of the eukaryotic cell. Nothing can function without the constant shuttling of intracellular components from place to place.Intracellular transport is orchestrated by a diverse constellation of molecular motor proteins that bind specific cargoes and convey them in a particular direction along cytoskeletal polymer tracks.Cytoskeletal polymer tracks are made up of intracellular network of filamentous structures.Essentially all eukaryotic cells contain 3 types of filamentous structures:actin filaments also k/a microfilamentsmicrotubules andintermediate filaments. four classes of intermediate filaments are found- Keratins, Vimentin- like proteins, Neurofilaments and Lamins.Axons are slender cylindrical processes and can extend for distances in excess of one meter in large animals, yet they are dependent on the cell body for the synthesis of many of their components. Materials destined for the axon are transported anterogradely, toward the axon tip, and materials destined to return are transported retrogradely, toward the cell body. This bidirectional transport process, known as axonal transport, is not fundamentally different from the pathways of macromolecular and membrane traffic that occur in all eukaryotic cells, but it is remarkable for its scale.Proteins and other molecules are transported along axons in association with distinct membranous and nonmembranous cargo structures that move at different rates. Membranous organelles move most rapidly, in the fast components of axonal transport, whereas cytoskeletal polymers and cytosolic protein complexes move more slowly, in the slow components.Membranous organelles like endocytic vesicles, lysosomes, golgi body derived vesicles are the principal cargoes of fast axonal transport. The many proteins, lipids, and polysaccharides that move along the axon at fast rates do so by virtue of their association with one or more subclasses of organelle or vesicle, either because they are sequestered within its lumen, embedded in its membrane, or bound to its surface.Membranous organelles move along both microtubule and microfilament tracks powered by molecular motor proteins (kinesins, dyneins, and myosins). Microtubules appear to be the principal tracks for long-range movementsalong the axis of the axon.Plus end-directed kinesin motors propel organelles along microtubules anterogradely, whereas dynein (and possibly also minus end-directed kinesin motors) propel organelles retrogradely.Non membranous cargoes like cytoskeletal polymers (microfilaments, microtubules and neurofilaments) and cytosolic protein complexes move more slowly, in the slow components.It was earlier believed that the fast and slow axonal transport are due to fundamentally distinct mechanisms of transport. But recent discoveries have shown that the underlying mechanism for both fast and slow transport is the same. They move at different rates due to difference in their duty ratio. The duty ratio is the proportion of time that a cargo structure spends actually moving. Thus the slow overall rate of movement of cytoskeletal filaments suggests that these structures move with a low duty ratio, spending most of their time not moving.Motile behavior of axonally transported cargoes Cargo structuresOverall rate (pulse labeling)Instantaneous rate (light microscopy)DirectionalityDuty ratioGolgi-derived vesicles (fast anterograde)200-400 mm/d~ (2-5 pm/s)1-5 pm/s-AnterogradeHighEndocytic vesicles, lysosomes, autophagosomes (fast retrograde)100-250 mm/d- (1-3 pm/s)1-3 pm/s-RetrogradeHighMitochondria<70 mm/d- (<0.8 pm/s)0.3-0.7 pm/s-BidirectionalIntermediateMicrofilaments, cytosolic protein complexes (slow component b)2-8 mm/d- (0.02-0.09 pm/s)UnknownUnknownUnknownMicrotubules, neurofilaments (slow component a)0.2-1 mm/d- (0.002-0.01 pm/s)0.3-1 pm/s-BidirectionalLow
Category:
Biochemistry
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