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Formation of Intermediate Filaments

Formation of Intermediate Filaments

Intermediate filaments are cytoskeletal proteins with higher tensile strength and flexibility than microfilaments and microtubules. Unlike the other two cytoskeletal proteins, intermediate filament formation lacks the enzymatic activity to hydrolyze nucleotides like ATP and GTP to generate energy for polymerization. Therefore, the formation of intermediate filaments is multistep self-assembly. The involvement of any accessory proteins in intermediate filament formation has not yet been reported.

The Structure of Intermediate Filaments

The Structure of Intermediate Filaments

The intermediate filaments are one of three widely studied cytoskeletal filaments. They are so named as their diameter (10 nm) is in between that of microfilaments (7 nm) and the microtubules (25 nm). These filaments are highly stable and can remain intact when exposed to high salt concentrations and detergents. These filaments are responsible for providing stability and mechanical support to the cells. They also help in cell adhesion and maintaining tissue integrity.
Intermediate filaments...

Assembly of Cytoskeletal Filaments

Assembly of Cytoskeletal Filaments

Cytoskeletal filaments are polymeric forms of smaller protein subunits. However, individual cytoskeletal filaments may easily disassemble or associate with other similar filaments to form rigid structures. Microfilaments, made of actin monomers, rely on actin-binding proteins to form bundles and create networks of individual actin filaments. Microtubules rely on microtubule-associated proteins (MAPs) to form sturdy cylindrical structures. However, the proteins involved in forming complex...

Disassembly of Intermediate Filaments

Disassembly of Intermediate Filaments

Intermediate filaments (IFs) do not undergo spontaneous disassembly. Enzymes, kinases, and phosphatases add and remove phosphates from specific sites to regulate their disassembly. The IF concentration in the cytoplasm also regulates the disassembly. If the concentration crosses a threshold, it activates the protein kinases in the vicinity, allowing the phosphorylation of IFs.
Keratin proteins, found at the cell periphery near cell junctions, undergo a cycle of assembly and disassembly. In Type...

Generation of Straight or Branched Actin Filaments

Generation of Straight or Branched Actin Filaments

The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. Formin-mediated assembly results in straight filaments, whereas Arp2/3 protein complex-mediated assembly results in branched actin filaments.
Arp2/3 Complex
Arp2/3 complex is a seven-subunit complex consisting of two proteins similar to actin- Arp2 and Arp3, and five other subunits that help keep Arp2 and Arp3 inactive. When required, the complex is...

Introduction to the Cytoskeleton

Introduction to the Cytoskeleton

Overview of the Cytoskeleton
The cytoskeleton is a network of protein filaments present within the cell, having three distinct filaments ̶ microfilaments, microtubules, and intermediate filaments. Each has characteristic features that distinguish them, including the dynamics of their assembly and disassembly, mechanical properties, polarity, and the type of molecular motors associated with them. Earlier, they were thought to be present only in eukaryotic cells; however, their homologs were...

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Related Experiment Video

Updated: Jun 15, 2026

Using Microfluidics and Fluorescence Microscopy to Study the Assembly Dynamics of Single Actin Filaments and Bundles

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Filament structure, organization, and dynamics in MreB sheets.

David Popp¹, Akihiro Narita, Kayo Maeda

¹ERATO Actin Filament Dynamics Project, Japan Science and Technology Corporation, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan.

The Journal of Biological Chemistry

|March 13, 2010

Summary

The bacterial shape-determining protein MreB forms complex sheet structures with superior mechanical properties, crucial for maintaining bacterial cell shape. Different nucleotide conditions yield distinct MreB filament arrangements for varied cellular functions.

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Area of Science:

Microbiology
Structural Biology
Biophysics

Background:

Bacterial shape is maintained by MreB, an actin homolog forming cable-like structures.
The precise molecular architecture of MreB cables remains uncharacterized.

Purpose of the Study:

To elucidate the molecular structure of MreB filaments and their assembly.
To understand how MreB structure relates to bacterial cell shape maintenance.

Main Methods:

Electron microscopy (EM) to visualize MreB structures.
Steady-state TIRF microscopy to study filament dynamics.
High-pressure small-angle X-ray scattering (SAXS) to assess polymer stability.

Main Results:

MreB forms multilayered sheets of diagonally interwoven filaments with ATP/GTP, suggesting enhanced mechanical properties.
MreB polymers exhibit single-stranded helical filaments in sheets, unlike crystal structures.
ADP/GDP induce parallel, linear protofilament cables, indicating nucleotide-dependent structural variations.
Sheet assembly is robust across various pH, ionic strength, and temperature conditions.

Conclusions:

MreB's sheet architecture likely contributes to bacterial cell shape maintenance.
The bacterial cell utilizes different nucleotides to generate diverse MreB filament structures for specific functions.