Microtubule Structure and Function Quiz

Microtubule Structure and Function

• Microtubules are hollow tubes made of tubulin and are 25 nm in diameter, consisting of 13 protofilaments in its wall.
• They can be divided into stable and labile groups, with stable microtubules found in cilia, flagella, centrioles, and basal bodies, and labile microtubules found in cytoplasmic microtubules and the mitotic spindle.
• Microtubules are polar structures with fast-growing (+) ends and slow-growing (-) ends, and the polarity is important in determining the direction of movement along microtubules.
• GTP regulates microtubule polymerization by binding to α and β tubulin, with GTP-bound dimers added to the plus end and hydrolyzed to GDP shortly after polymerization, leading to dynamic instability.
• Stabilization of microtubules is regulated by MAPs (Microtubule Associated Proteins), which organize microtubules, affect their stability, and can either stabilize or destabilize them.
• Cytoplasmic microtubules have functions in intracellular transport, organelle stability, the formation of the mitotic spindle, and the production of cellular structures.
• Drugs like colchicine, colcemid, vincristine, and vinblastine bind tubulin and inhibit microtubule polymerization, while taxol stabilizes microtubules.
• Microtubule motor proteins include kinesin, which transports cytoplasmic vesicles toward the (+) end, and dynein, which transports cytoplasmic vesicles toward the (-) end and is responsible for ciliary movement.
• The properties of kinesin include a structurally similar head domain to myosin, and over 100 known kinesin proteins with different tail sequences moving different cargo types.
• Dynein is a 2000 kD protein with motor domains that bind ATP and move along microtubules toward minus ends, resulting in bending.
• Centrosomes serve as the microtubule organizing center in cells, playing a big role in determining the intracellular organization of microtubules and serving as the initiation role of microtubule assembly.
• Centrosomes are located adjacent to the nucleus, and they consist of centrioles and are responsible for the outward growth of microtubules towards the cell periphery.

Microfilaments and Intermediate Filaments: Key Facts

• Microfilaments are composed of actin and play a crucial role in cellular functions such as cytoplasmic streaming and amoeboid motion.
• The cell cortex contains perpendicular actin, while the streaming portion has parallel actin, facilitating cytoplasm movement.
• Microfilaments can assemble into various structures within a cell, including labile structures like cell cortex and stress fibers, and stable structures like microvillus and sarcomer.
• Actin, with a diameter of 7nm, constitutes 5-10% of total cellular protein and has a polarized structure with + and - ends.
• Factors affecting microfilament formation include fungus toxins like cytochalasin B and proteins like profilin and thymosin β4.
• Capping proteins and drugs like cytochalasin and phalloidin can inhibit actin formation and cell movements.
• Actin-binding proteins like fimbrin, villin, filamin, and spectrin play essential roles in linking and cross-linking actin filaments.
• Microfilament motor proteins like myosin I, myosin II, and myosin V convert chemical energy to mechanical energy and are crucial for cellular functions.
• Functions of actin filaments include providing mechanical strength to cells, linking transmembrane proteins to cytoplasmic proteins, and generating locomotion in cells.
• Intermediate filaments, with a diameter of about 10 nm, help maintain cell shape and organelle position and are made of protein subunits such as keratin.
• Intermediate filaments are biochemically heterogeneous, have great tensile strength, do not have intrinsic polarity, and lack motors that use them as tracks.
• They extend throughout the cytoplasm and line the inner nuclear envelope of interphase animal cells.