A molecular model for LINC complex regulation: activation of SUN2 for KASH binding

Abstract

Linkers of the nucleoskeleton and cytoskeleton are key molecular complexes that span the nuclear envelope (NE) and provide a direct linkage between the nucleoskeleton and cytoskeleton. Two major components of these complexes are members of the SUN and KASH protein families that interact in the perinuclear space to allow the transmission of mechanochemical signals across the NE. Structural details of the mammalian SUN domain protein SUN2 have established that SUN2 must form a trimer to bind to KASH, and that this trimerization is mediated through two predicted coiled-coil regions of the protein, CC1 and CC2, which precede the SUN domain. Recent crystallographic data suggest that CC2-SUN formed an unexpected autoinhibited monomer unable to bind to KASH. These structural insights raise the question of how full-length SUN2 transitions from a monomer to a trimer inside the NE. In this study we used a computational approach to model a fragment of SUN2 containing CC1, CC2, and the SUN domain. We observed the dynamics of these modeled structures using ∼1 μs molecular dynamics simulations and showed that the interplay between CC1 and CC2 may be sufficient for the release of CC2-SUN2 from its autoinhibited state. Additionally, using our models and gel filtration analysis, we show the involvement of an E452 residue on CC1 in the monomer–­trimer transition of SUN2. Intriguingly, mutations in this residue have been seen in muscular dystrophy–associated SUN2 variants. Finally, we propose a Ca2+-dependent monomer–trimer transition of SUN2.

Signal Transduction across the Nuclear Envelope: Role of the LINC Complex in Bidirectional Signaling

“…The LINC complex, which is localized at the NE, acts in a variety of signaling pathways between the cytoplasm and the nucleus. Currently the mechanisms responsible for regulating the assembly of functional LINC complexes remain poorly defined, although several lines of evidence identify several candidates for LINC complex regulators including intraluminal calcium, the redox environment of the NE, torsinA, and ubiquitinylation [63,106,107,108]. Finally, an analogy between FAs and LINC complexes can be drawn.…”

SUN/KASH interactions facilitate force transmission across the nuclear envelope

“…Mutating residues at the interface between this alpha helix and the KASH-lid restored the KASH-binding capacity of the SUN domain [35]. Interestingly, the KASH-lid is released from the autoinhibited state in the presence of the trimerization coiled-coil in Molecular Dynamic simulations [36]. Both the crystal model and simulations indicate that a calcium ion can occupy a cation loop formed in the extended coiled-coil domain of the SUN protein, which might play a role in activating the LINC complex.…”

“…When a conserved residue (E452) in the mouse SUN2 trimerization coil is not occupied by Ca 2+ , it interacts with the SUN domain and potentially inactivates LINC complex formation. Both the E452 mutation in SUN2 and Ca 2+ concentration changes monomer to trimer ratios in gel filtration assays [36]. Notably, the E452D mutation in human SUN2 was identified in muscular dystrophy patients [34][35][36].…”

“…Both the E452 mutation in SUN2 and Ca 2+ concentration changes monomer to trimer ratios in gel filtration assays [36]. Notably, the E452D mutation in human SUN2 was identified in muscular dystrophy patients [34][35][36]. Further in vivo studies are needed to demonstrate the significance and mechanisms for the SUN protein activation, which is essential to form LINC complex and transfer force across the nuclear envelope.…”

Chain reaction: LINC complexes and nuclear positioning

“…Clearly, CC2 must be able to switch reversibly between its inhibitory conformation and its extended conformation permissive for trimerisation and KASH binding. What is not clear is whether this switching may still occur after trimer formation 74 . If it can occur, then it opens up a range of regulatory possibilities.…”