Intermediate filaments (IF) are the most flexible structures of the cytoskeletal system. They are intermediate in diameter compared to the larger microtubules and smaller actin filaments. There are at least 65 different human IFs and they range in size from about 9-11 nm. They are found only in eukaryotic cells, although IF like proteins are present in prokaryotes.

Intermediate filaments give integrity to the nuclear envelope and cell surface membrane. One type of IF, keratin, makes up hair, nails, horns, and reptilian scales. Another, vimentin, anchors cell organelles and is found attached to the nucleus, endoplasmic reticulum, and mitochondria.

Selected Publications:

Structural analysis of vimentin and keratin intermediate filaments by cryo-electron tomography.

Exp Cell Res. 2007 Jun 10;313(10):2217-27. Epub 2007 Apr 11.

Norlén L, Masich S, Goldie KN, Hoenger A.

Intermediate filaments are a large and structurally diverse group of cellular filaments that are classified into five different groups. They are referred to as intermediate filaments (IFs) because they are intermediate in diameter between the two other cytoskeletal filament systems that is filamentous actin and microtubules. The basic building block of IFs is a predominantly alpha-helical rod with variable length globular N- and C-terminal domains. On the ultra-structural level there are two major differences between IFs and microtubules or actin filaments: IFs are non-polar, and they do not exhibit large globular domains. IF molecules associate via a coiled-coil interaction into dimers and higher oligomers. Structural investigations into the molecular building plan of IFs have been performed with a variety of biophysical and imaging methods such as negative staining and metal-shadowing electron microscopy (EM), mass determination by scanning transmission EM, X-ray crystallography on fragments of the IF stalk and low-angle X-ray scattering. The actual packing of IF dimers into a long filament varies between the different families. Typically the dimers form so called protofibrils that further assemble into a filament. Here we introduce new cryo-imaging methods for structural investigations of IFs in vitro and in vivo, i.e., cryo-electron microscopy and cryo-electron tomography, as well as associated techniques such as the preparation and handling of vitrified sections of cellular specimens.

Dissecting the 3-D structure of vimentin intermediate filaments by cryo-electron tomography

J Struct Biol. 2007 Jun;158(3):378-85. Epub 2006 Dec 28.

Goldie KN, Wedig T, Mitra AK, Aebi U, Herrmann H, Hoenger A.

Vimentin polymerizes via complex lateral interactions of coiled-coil dimers into long, flexible filaments referred to as intermediate filaments (IFs). Intermediate in diameter between microtubules and microfilaments, IFs constitute the third cytoskeletal filament system of metazoan cells. Here we investigated the molecular basis of the 3-D architecture of vimentin IFs by cryo-electron microscopy (cryo-EM) as well as cryo-electron tomography (Cryo-ET) 3-D reconstruction. We demonstrate that vimentin filaments in cross-section exhibit predominantly a four-stranded protofibrilar organization with a right-handed supertwist with a helical pitch of about 96 nm. Compact filaments imaged by cryo-EM appear surprisingly straight and hence appear very stiff. In addition, IFs exhibited an increased flexibility at sites of partial unraveling. This is in strong contrast to chemically fixed, negatively stained preparations of vimentin filaments that generally exhibit smooth bending without untwisting. At some point along the filament unraveling may be triggered and propagates in a cooperative manner so that long stretches of filaments appear to have unraveled rapidly in a coordinated fashion.