Hoenger Lab
University of Colorado at Boulder
Department of Molecular, Cellular and Developmental Biology

Structural and Functional Investigations
into Cytoskeletal Assemblies by
Cryo Electron Microscopy
and 3D Image Analysis

Research:: Intermediate Filaments

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.

This picture is an cryo electron micrograph of vimentin intermediate filaments. These samples have been embedded in ice.

The arrows show the flexible regions where the IFs have unraveled from a tighter confiuration.

The basic building block of vimentin like all other IFs is a monomer with a central alpha helical region flanked on one end by a non helical, amino, head domain and on the other side, by a carboxy tail. These monomers twist together to form dimers, two dimers form a tetramers, and eight tetramers make up the ULF, unit length filament. Unit length filaments assemble into full length IFs.


University of Colorado, MCDB, 347 UCB, Boulder, CO 80309-0347