Abstract
Given the role of intermediate filaments (IFs) in normal cell physiology and scores of IF-linked diseases, the importance of understanding their molecular structure is beyond doubt. Research into the IF structure was initiated more than 30 years ago, and some important advances have been made. Using crystallography and other methods, the central coiled-coil domain of the elementary dimer and also the structural basis of the soluble tetramer formation have been studied to atomic precision. However, the molecular interactions driving later stages of the filament assembly are still not fully understood. For cytoplasmic IFs, much of the currently available insight is due to chemical cross-linking experiments that date back to the 1990s. This technique has since been radically improved, and several groups have utilized it recently to obtain data on lamin filament assembly. Here, we will summarize these findings and reflect on the remaining open questions and challenges of IF structure. We argue that, in addition to X-ray crystallography, chemical cross-linking and cryoelectron microscopy are the techniques that should enable major new advances in the field in the near future.
Highlights
Laboratory for Biocrystallography, KU Leuven, 3000 Leuven, Belgium; Institute of Microbiology of the Czech Academy of Sciences, 14220 Prague, Czech Republic
While crystallography remains the main source of atomic detail of the intermediate filaments (IFs) rod structure, an important complementary approach is the use of electron paramagnetic resonance on site-directed spin-labelled samples (SDSL-EPR) [26]
(~40%) of the detected cross-links could not dimer be explained by the has been achieved inthat establishing the three-dimensional structure of the elementary dimer
Summary
Intermediate filaments (IFs) together with actin microfilaments (MFs) and microtubules (MTs) are the three main cytoskeletal filament systems found in metazoan animals. A signature feature of IF proteins is their central α-helical ‘rod’ domain Structural organization of this domain is conserved across all IF types. Of nuclear lamins is very distinct from that of cytoplasmic IFs. As demonstrated in vitro, lamin dimers have the capacity to associate longitudinally to form longer head-to-tail threads, which can further associate [6,17,18,19]. During mitosis, site-specific phosphorylation plays a key role in IF disassembly [23,24,25] Of note, it is the dynamic character of IFs that further complicates their structural studies. We will discuss the current knowledge on the structure of cytoplasmic and nuclear IFs, especially focusing on the data available on the interdimer interactions driving the filament assembly.
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