SUN domain proteins are conserved proteins of the nuclear envelope and key components of the LINC complexes (linkers of the nucleoskeleton and the cytoskeleton). Previous studies have demonstrated that the testis-specific SUN domain protein SUN4 is a vital player in the directed shaping of the spermatid nucleus. However, its molecular properties relating to this crucial function have remained largely unknown and controversial data for the organization and orientation of SUN4 within the spermatid nuclear envelope have been presented so far. Here we have re-evaluated this issue in detail and show robust evidence that SUN4 is integral to the inner nuclear membrane, sharing a classical SUN domain protein topology. The C-terminal SUN domain of SUN4 localizes to the perinuclear space, while the N-terminus is directed to the nucleoplasm, interacting with the spermiogenesis-specific lamin B3. We found that SUN4 forms heteromeric assemblies with SUN3 in vivo and regulates SUN3 expression. Together, our results contribute to a better understanding of the specific function of SUN4 at the spermatid nucleo-cytoplasmic junction and the process of sperm-head formation.

SUN domain proteins are conserved proteins of the nuclear envelope and key components of the LINC complexes (linkers of the nucleoskeleton and the cytoskeleton). Previous studies have demonstrated that the testis-specific SUN domain protein Sun4 is a vital player in spermatogenesis, critically involved in the directed shaping of the spermatid nucleus. Its molecular properties relating to this crucial function, however, have remained largely unknown. Previous studies presented quite controversial data for the general organization and orientation of Sun4 within the spermatid nuclear envelope. In the present study, we have re-evaluated this issue in detail and present new robust data on the Sun4 topology and its interactions at the nucleo-cytoplasmic junction. We identified Sun4 as an integral protein of the inner nuclear membrane, sharing a classical SUN domain protein topology. Similar to other SUN domain proteins, the C-terminal SUN domain of Sun4 locates to the perinuclear space and the N-terminus is directed to the nucleoplasm, where it interacts with the spermiogenesis specific Lamin B3. We found that Sun4 in its natural environment forms heteromeric assemblies with Sun3 and, beyond this, it is crucially involved in the regulation of Sun3 expression. Together, our results contribute to a better understanding of the specific function of Sun4 at the spermatid nucleo-cytoplasmic junction and the entire process of sperm-head formation.Summary statementIn our current study, we have analyzed in detail the biochemical and dynamic properties of the testis-specific SUN domain protein Sun4 and we provide novel insights into its interaction behavior at the spermatid nucleo-cytoplasmic junction.