Filament assembly of the C. elegans lamin in the absence of helix 1A

Abstract

Lamins are the major constituent of the nuclear lamina, a protein meshwork underlying the inner nuclear membrane. Nuclear lamins are type V intermediate filaments that assemble into ~3.5 nm thick filaments. To date, only the conditions for the in vitro assembly of Caenorhabditis elegans lamin ( Ce -lamin) are known. Here, we investigated the assembly of Ce -lamin filaments by cryo-electron microscopy and tomography. We show that Ce -lamin is composed of ~3.5 nm protofilaments that further interact in vitro and are often seen as 6–8 nm thick filaments. We show that the assembly of lamin filaments is undisturbed by the removal of flexible domains, that is, the intrinsically unstructured head and tail domains. In contrast, much of the coiled-coil domains are scaffold elements that are essential for filament assembly. Moreover, our results suggest that Ce -lamin helix 1A has a minor scaffolding role but is important to the lateral assembly regulation of lamin protofilaments.

Molecular structure of soluble vimentin tetramers

“…Previous cryoelectron microscopy (cryoEM) studies revealed that mature vimentin IFs typically contained four octameric protofibrils with right-handed supertwisting, while considerable structural variability was also observed 14 , 15 . Further cryoEM observations of several IF classes were reported recently 16 – 19 . However, preparations of such filaments, both extracted from cells and assembled in vitro, suffer from considerable heterogeneity, which has limited the resolution of these studies.…”