Structural analysis of the ternary complex between lamin A/C, BAF and emerin identifies an interface disrupted in autosomal recessive progeroid diseases

Abstract

Lamins are the main components of the nucleoskeleton. Whereas their 3D organization was recently described using cryoelectron tomography, no structural data highlights how they interact with their partners at the interface between the inner nuclear envelope and chromatin. A large number of mutations causing rare genetic disorders called laminopathies were identified in the C-terminal globular Igfold domain of lamins A and C. We here present a first structural description of the interaction between the lamin A/C immunoglobulin-like domain and emerin, a nuclear envelope protein. We reveal that this lamin A/C domain both directly binds self-assembled emerin and interacts with monomeric emerin LEM domain through the dimeric chromatin-associated Barrier-to-Autointegration Factor (BAF) protein. Mutations causing autosomal recessive progeroid syndromes specifically impair proper binding of lamin A/C domain to BAF, thus destabilizing the link between lamin A/C and BAF in cells. Recent data revealed that, during nuclear assembly, BAF’s ability to bridge distant DNA sites is essential for guiding membranes to form a single nucleus around the mitotic chromosome ensemble. Our results suggest that BAF interaction with lamin A/C also plays an essential role, and that mutations associated with progeroid syndromes leads to a dysregulation of BAF-mediated chromatin organization and gene expression.

Protein structural and mechanistic basis of progeroid laminopathies

“…BAF directly binds to the LEM domain of the inner nuclear transmembrane proteins emerin [84] and LAP2ß [85], as well as to the Igfold of lamin A/C. Analysis of the 3D structure of the lamin A/C-BAF complex reveals that one Igfold domain binds to one BAF dimer [81]. Furthermore, almost all of the lamin A/C residues mutated in homozygous APS are on the surface binding to BAF (Fig.…”

“…The highly evolutionarily conserved protein BAF directly interacts with lamin A/C [80,81]. BAF is a small, 89-amino acid protein that forms obligate dimers, each subunit of which binds to doublestranded DNA (dsDNA) allowing BAF to bridge two strands of dsDNA [82,83].…”

“…The affinity decrease correlates with the severity of the disease. For example, mutation Arg435Cys causing the most severe disease (a progeroid disorder with RD-like symptoms) completely abolishes the binding, as measured by isothermal titration calorimetry, whereas mutations Arg471Cys and Ala529Val causing less severe progeroid disorders only cause a fivefold decrease in affinity [81]. The Arg435Cys mutant is also detected in lower amounts in patient cells compared to an unaffected control, which might contribute to the severity of the disease [87].…”

“…Consistently, a homozygous BANF1 mutation (c.34G>A) leading to an Ala12Thr amino acid substitution causes the recessive progeroid disorder NGPS [13,88]. The amino acid change impairs BAF binding to lamin A/C in vitro [81] and in cells [89]. NGPS partially phenocopies HGPS but patients do not have early-onset cardiovascular disease and therefore have a relatively long lifespan [13].…”

A polarized nucleus-cytoskeleton-ECM connection controls collective migration and cardioblasts number inDrosophila

“…Recent findings have shown that deleting components of the LINC complex abolished control of endoreplication in myofibers nuclei by affecting the chromatin regulator BAF leading to an increase DNA content 36 . Mutations in nesprin-1 have been shown to reduce levels of Lamin A/C 37 , a known partner of BAF 38 , leading to changes in chromatin structure and aberrant expression of genes involved in differentiation 39 . We also observed a loss of Lamin C in CBs upon MSP-300 loss of function (data not shown).…”