List the steps of assembly of intermediate filaments

Intermediate filaments (IFs) are strong, rope-like structures in the cytoskeleton of animal cells. They provide mechanical strength and help maintain the shape and stability of the cell. Unlike microtubules and actin filaments, intermediate filaments are not involved in movement but are mainly known for their structural support. The process of their assembly is step-wise and highly organized.

There are six main steps involved in the assembly of intermediate filaments.

Step 1: Formation of Monomers

The basic building blocks of intermediate filaments are protein monomers. Each monomer has a long central alpha-helical rod domain and short non-helical head and tail regions. These monomers are soluble in the cytoplasm and exist freely before assembly begins.

Step 2: Formation of Coiled-Coil Dimers

Two monomers come together and wrap around each other in parallel to form a coiled-coil dimer. In this dimer, both monomers are aligned in the same direction (parallel) and their alpha-helices interact to form a stable structure.

Step 3: Formation of Tetramers

Next, two dimers come together in an antiparallel and staggered arrangement to form a tetramer. This is a very important step because this tetramer is the main unit used for further filament formation. Each tetramer has four polypeptide chains, arranged in such a way that the ends of the filaments do not have polarity, unlike microtubules.

Step 4: Formation of Protofilaments

Several tetramers align side by side, overlapping slightly, to form protofilaments. These are short linear structures made from multiple tetramers. The bonding between the tetramers is mainly through weak non-covalent interactions.

Step 5: Formation of Protofibrils

Four protofilaments laterally associate with each other to form a protofibril. This lateral packing provides strength and thickness to the structure.

Step 6: Formation of Mature Intermediate Filament

Finally, about eight protofilaments twist around each other to form a mature intermediate filament. This final filament is about 10 nanometers in diameter and is very strong, flexible and resistant to mechanical stress. This is the complete and functional form found in the cytoplasm.

Throughout this process, no ATP or GTP is required, which makes intermediate filament assembly different from microtubules or actin filaments. Also, since the tetramers are antiparallel, the final filament has no polarity.
Intermediate filaments (IFs) are strong, rope-like structures in the cytoskeleton of animal cells. They provide mechanical strength and help maintain the shape and stability of the cell. Unlike microtubules and actin filaments, intermediate filaments are not involved in movement but are mainly known for their structural support. The process of their assembly is step-wise and highly organized. There are six main steps involved in the assembly of intermediate filaments.







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