SUN1 splice variants, SUN1_888, SUN1_785, and predominant SUN1_916, variably function in directional cell migration

Abstract

The LINC complex is a multifunctional protein complex that is involved in various processes at the nuclear envelope, such as nuclear migration, mechanotransduction and chromatin tethering in the meiotic phase. However, it remains unknown how these functions are regulated in different cell contexts. An inner nuclear membrane component of the LINC complex, SUN1, is ubiquitously expressed. The human SUN1 gene produces over 10 variants by alternative splicing. Although functions of SUN1 are relatively well characterized, functional differences among SUN1 splice variants are poorly characterized. LINC complex components are associated with a wide range of human diseases; therefore, it is important to understand the functional diversity among SUN1 splice variants. Here, we identified a novel human SUN1 splice variant, SUN1_888. overexpression of the SUN1 splice variants, SUN1_888 or SUN1_785, but not the predominant isoform, SUN1_916, activated directional cell migration. Knockdown of SUN1_888 suppressed cell migration; in contrast depletion of SUN1_916 activated cell migration. In addition, all of investigated SUN1 splicing variants rescued cell migration in SUN1 knock out cell. These results indicate that redundant and non-redundant functions of SUN1 splice variant in directional cell migration and suggest that variable LINC complexes with distinct task may exit. Furthermore, in contrast to previous studies, we showed association between SUN1 and B-type lamins. Interestingly, B-type lamin preferentially interacts with SUN1 but not SUN2. These results suggest that tissue-specific SUN1 variants variably interact with nucleoplasmic partners and allow variable assembly of LINC complexes that can be assigned to distinct tasks.

Implications for Diverse Functions of the LINC Complexes Based on the Structure

“…Moreover, SUN1–SUN2 heterotrimer formation is feasible for the following two reasons. First, biochemical analyses have shown that SUN1 and SUN2 can form a hetero-oligomer [11,28,49,55,56,57], and second, the molecular organization of SUN 1 and SUN2, such as the length of the coiled-coil domains or the SUN domain, is similar. Thus, four kinds of SUN protein trimers are possible (Figure 2B).…”

“…Likewise, SUN1 may exist in dimers or tetramers, not trimers in cells under some circumstances [55]. In addition, since each gene encoding SUN and nesprin proteins produces a wide range of splicing variants, and the expression level of variant proteins is tissue-dependent, the compositional nature of SUN–KASH hexamers significantly varies among tissues [10,11,12,21,28,58]. …”

“…The nucleoplasmic region of both SUN1 and SUN2 interacts with lamin A, whereas the interaction with B-type lamins appears to be relatively weak [11,16]. However, as mentioned above, a recent solution binding assay and yeast two-hybrid analysis revealed that both lamin B1 and lamin B2 interact with SUN1 but not with SUN2 [28]. …”

“…Mammalian SUN proteins interact with A-type lamin, whereas their binding to B-type lamins is generally considered to be very weak [11,16]. However, we recently demonstrated that B-type lamins also interact with SUN1 but not with SUN2 [28]. …”

“…The LINC complex performs diverse functions, including nuclear shaping and positioning [29], maintenance of the centrosome–nucleus connection [30], DNA repair [31,32], nuclear membrane spacing [11], cell migration [28,29,33,34,35], and moving chromosomes within the nucleus during meiosis [36]. In addition, lamins play various roles such as maintenance of nuclear integrity, cell cycle regulation, mechanotransduction, cellular signalling, and DNA repair.…”