Force transmission and SUN-KASH higher-order assembly in the LINC complex models

The linker of the nucleoskeleton and cytoskeleton (LINC) complex comprises SUN (Sad-1 and UNC-84) and KASH (Klarsicht, ANC-1, SYNE homology) domain proteins, whose conserved interactions provide a physical coupling between the cytoskeleton and the nucleoskeleton, thereby mediating the transfer of physical forces across the nuclear envelope. The LINC complex can perform distinct cellular functions by pairing various KASH domain proteins with the same SUN domain protein. Recent studies have suggested a higher-order assembly of SUN and KASH instead of a more widely accepted linear trimer model for the LINC complex. In the present study, we use molecular dynamics simulations to investigate the mechanism of force transfer across the two proposed models of LINC complex assembly, namely the 3:3 linear trimer model and the 6:6 higher-order model. Employing steered molecular dynamics simulations with various structures using forces at different rates and directions, we examine the structural stability of the two models under various biologically relevant conditions. Our results suggest that both models can withstand and transfer significant levels of force while retaining their structural integrity. However, the force response of various SUN KASH assemblies depended on the force direction and pulling rates. Slower pulling rates resulted in higher mean square fluctuations of the 3:3 assembly compared to the fast pulling. Interestingly, the 6:6 assembly tends to provide an additional range of motion flexibility and might be more suitable for describing the interaction between SUN and KASH under compressive and shear forces. These findings offer insights into how the SUN and KASH proteins maintain the structural integrity of the nuclear membrane.