The role of microtubules in intracellular transport variety

The role of microtubules in intracellular transport

variety of cell movements by microtubules • Microtubules are responsible for a variety of cell movements, including • the intracellular transport and positioning of membrane vesicles and organelles • the separation of chromosomes at mitosis, • and the beating of cilia and flagella.

microtubules in intracellular transport Microtubules act as pathways for transporting within the interphase cells. Two major families of motor proteins Kinesin- Dynein to be involved in cargo transport along the microtubule.

Kinesin-1 structure • Kinesin purified from squid axon, consisting of a protein with two heavychains that each of them bound to a small chain. • head domain, connected to microtubules and ATP and responsible for kinesin motor activity. Connective domain is essential in forward moving. stalk domin is involved in the dimerization of the two heavy chains. The tail domin is responsible for binding to receptors in the membrane.

Kinesin functions Kinesin causes vesicles to move from negative to positive end microtubule. (anterograde direction ) and it needs ATP to do that. In addition to the role of Kinesin-1 in regulating axonal transport, other types of studied Kinesin are involved in the transfer of organelles m. RNA and chromosomes.

Dynein In addition to Kinesin motors, which generally transmit positive and. forward microtubules, cells have another type of motor protein called dynein that transfers cargoes in a negative end of microtubules( reversiblereterograde direction) •

Dynein structure It is a very large motor protein and consists of two large subunits, two • intermediate subunits, and two small subunits. Its heavy chain consists of a trunk and a globular -head domains that connect to microtubule and has an ATPase activity. The tail domain connected to cargo.

dynein and dynactin transport cargos • Unlike kinesin 1, dynein alone is not capable of transporting substances, and requires a large protein complex called dynactin that causes dynein bind to cargo and regulates the activity of the dynein.

Microtubule-dependent cilia and flagella structures • The cilia and flagella are microtubule-dependent and membrane-bound surface structures which branch out from the surface of many protozoa and animal cells. Cilia and flagella move liquid past the surface of the cell. For single cells, such as sperm, this enables them to swim. For cells anchored in a tissue, like the epithelial cells lining our air passages, this moves liquid over the surface of the cell.

The structure of the cilia and flagella • The cilia and flagella have a cluster of central microtubules called axoneme, which consists of 9 double microtubules that enclose a pair of central microtubules that are structurally distinct.

axoneme structure • The axoneme structure is maintained by three transverse interface protein assemblies. • Two single central microtubules are connected by alternating bridges such as ladder stairs. • Another set of interface protein , made of nexin protein that bind to the adjacent double microtubules. • Radial spokes extend from each of the outer microtubules to the central pair.

The mechanism of flexion of the flagella and cilia • The most important motor protein present in the flagella and cilia structures is called axonal dynein. There are two rows of dynein motors intercalate along each of outer microtubules that are termed the inner arm and outer arm dynein. In fact, the interaction between these dynein arms with the adjacent B tubule causes the flagella and the cilia flexion.