Cytoskeleton

Desmin is the intermediate filament (IF) protein that polymerizes to form the desniin IF of skeletal muscle. It is found in smooth, skeletal, and cardiac muscle. The desmin IF links desmosomes of cardiac muscle, thus serving to stabilize cell junctions. Actin is the protein of microfilaments-the smallest diameter filament of the cytoskeleton. Actinin is an actin-associated protein (an actin-bundling protein) that serves to cross4ink actin bundles and anchor actin filaments to the cell membrane. Clathrin forms a basket or cage around endocytotic vesicles. Vimentin is a widely distributed IF protein present in many cells of mesodermal origin.

C i.e. Neurofilament

Axonal transport of membranous & non membranous cargoes

• It is now known that intracellular transport is coordinate by molecular motor proteins that bind cargoes and convey them in a particular direction along cytoskeletal polymer tracks. This includes every type of membranous organelle and transport vesicle (membranous cargoes), as well as nonmembranous cargoes such as cytoskeletal polymers, cytosolic protein complexes, ribosomes, and messenger RNAs.
• Membranous organelles move most rapidly, in the fast components of axonal transport, whereas nonmembranous cytoskeletal polymers and cytosolic protein complexes move more slowly, in the slow components. Recent studies suggest that slow axonal transport is generated by fast motors and that the slow rate is due to rapid movements interrupted by prolonged pauses.
• 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
• Membranous organelles 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.
• Both neurofilaments and microtubules move at fast rates, approaching the rate of movement of membranous organdies, but the average rate of movement is slow because the movements are both infrequent and bidirectional. Thus, the overall speed and direction of neurofilament and microtubule movement is a temporal summation of anterograde and retrograde movements and pauses, perhaps not fundamentally dissimilar from the behavior of mitochondria in axons described above. As is the case for mitochondria, the slow overall rate of movement of neurofilaments and microtubules suggests that these structures move with a low duty ratio, spending most of their time not moving.
• The rapid rate of movement of neurofilaments and microtubules in axons indicates that they are transported by fast motors, perhaps similar or identical to motors that move membranous organelles. Several lines of evidence suggest that dynein may transport axonal microtubules anterogradely, perhaps relative to the microfilament matrix, and that dynein and kinesin may transport axonal neurofilaments bidirectionally along microtubules by the same mechanism that is thought to move vimentin along microtubules in nonneuronal cells
• Cytoskeletal proteins have been the exclusive focus of studies on slow axonal transport in recent years, but it is important to remember that several hundred other proteins also move in this rate group, representing the entire spectrum of cytosolic proteins that comprise axoplasm. Some examples include proteins involved in vesicle dynamics such as clathrin and synapsin; regulatory proteins such as calmodulin; metabolic enzymes such as creatine kinase, aldolase, and enolase; cytoskeletal proteins such as spectrin, tau, and dynactin; and motor proteins such as dynein and myosin
• Cytoskeletal polymer tracks are made up of intracellular network of filamentous structure. All eukeryotic cells have 3 types of filaments – Microfilaments (actin filaments)

• – Microtubules (tubulin filaments)

  – Intermediate filaments has 4 classes: keratins, neurofilaments, lamins and vimentin like proteins

D i.e. GTP not required

Ans. is ‘b’ i.e., Actin

Ans. is `c’ i.e., Tubulin