Fluorescence fluctuation spectroscopy reveals differential SUN protein oligomerization in living cells

Linker-of-nucleoskeleton-and-cytoskeleton (LINC) complexes are conserved molecular bridges within the nuclear envelope that mediate mechanical force transmission into the nucleoplasm. The core of a LINC complex is formed by a transluminal interaction between the outer and inner nuclear membrane KASH and SUN proteins, respectively. Mammals encode six KASH proteins and five SUN proteins. Recently, KASH proteins were shown to bind to the domain interfaces of trimeric SUN2 proteins in vitro. However, neither the existence of SUN2 trimers in living cells nor the extent to which other SUN proteins conform to this assembly state have been tested experimentally. Here we extend the application of fluorescence fluctuation spectroscopy to quantify SUN protein oligomerization in the nuclear envelopes of living cells. Using this approach, we demonstrate for the first time that SUN2 trimerizes in vivo and we demonstrate that the in vivo oligomerization of SUN1 is not limited to a trimer. In addition, we provide evidence to support the existence of potential regulators of SUN protein oligomerization in the nuclear envelope. The differential SUN protein oligomerization illustrated here suggests that SUN proteins may have evolved to form different assembly states in order to participate in diverse mechanotransduction events.