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Intermediate Filaments - Vimentin
Intermediate filaments (IFs) are key components of the cytoskeleton in higher eukaryotic cells. The elementary IF ‘building block’ is an elongated coiled-coil dimer consisting of four consecutive a-helical segments. The segments 1A and 2B include highly conserved sequences and are critically involved in IF assembly. Based on the crystal structures of three human vimentin fragments at 1.4?2.3 Å resolution (PDB entries 1gk4, 1gk6 and 1gk7), we have established the molecular organization of these two segments. The fragment corresponding to segment 1A forms a single, amphipatica-helix, which is compatible with a coiled-coil geometry. While this segment might yield a coiled coil within an isolated dimer, monomeric 1A helices are likely to play a role in specific dimer-dimer interactions during IF assembly. The 2B segment reveals a double-stranded coiled coil, which unwinds near residue Phe351 to accommodate a ‘stutter’. A fragment containing the last seven heptads of 2B interferes heavily with IF assembly and also transforms mature vimentin filaments into a new kind of structure. These results provide the first insight into the architecture and functioning of IFs at the atomic level (Reference).
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Nearly all intermediate filament (IF) proteins exhibit a highly conserved amino acid motif (YRKLLEGEE) at the C-terminal end of their central alpha-helical rod domain. Our results indicated that the YRKLLEGEE-motif is crucial for the formation of authentic tetrameric complexes and in addition, during assembly, for control of filament width rather than elongation. The structure of the YRKLLEGEE domain, which was determinded from crystals of peptides containing the last 28 amino acids of coil 2B, chimerically fused at its amino-terminal end to the 31 amino acid-long leucine zipper domain of the yeast transcription factor GCN4 to facilitate appropriate coiled-coil formation. The atomic structure shows that starting from Tyr400 the two chains, rather than staying coiled-coil, slowly but definitely separate and past Gly406 become partially disordered bending away from the coiled-coil axis. (Reference)
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Cortexillin I
The X-ray crystal structure of the 18-heptad repeat alpha-helical coiled coil domain of the actin bundling protein cortexillin I from Dictyostelium discoideum is a tightly packed parallel two-stranded alpha-helical coiled coil. It harbors a distinct 14-residue long sequence motif that is absolutely required for coiled-coil formation, and which is a prerequisite for the assembly of cortexillin I. The atomic structure reveals novel types of ionic coiled-coil interactions, and in particular, it shows that a characteristic inter- and intra-helical salt bridge pattern in combination with the hydrophobic interactions ocurring at the dimer interface is the key structural feature of its coiled-coil trigger site. This knowledge can be used in the de novodesign of a-helical coiled coils for applications such as two-stage drug targeting and delivery systems, coiled coils as templates for combinatorial helical libraries for drug discovery and as synthetic carrier molecules. (Reference)