Department of Cell and Developmental Biology, Biocenter, University of Würzburg, Am Hubland, 97074 Würzburg, Germany.
J Cell Sci. 2023 Apr 1;136(7). doi: 10.1242/jcs.260155. Epub 2023 Mar 27.
SUN domain proteins are conserved proteins of the nuclear envelope and key components of the LINC complexes (for 'linkers of the nucleoskeleton and the cytoskeleton'). Previous studies have demonstrated that the testis-specific SUN domain protein SUN4 (also known as SPAG4) 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, whereas 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 结构域蛋白是核膜的保守蛋白，也是 LINC 复合物（“核骨架和细胞骨架的连接物”）的关键组成部分。先前的研究表明，睾丸特异性 SUN 结构域蛋白 SUN4（也称为 SPAG4）是精子细胞核定向成形的重要参与者。然而，与其关键功能相关的分子特性在很大程度上仍不清楚，到目前为止，有关 SUN4 在精子核膜中的组织和定向的数据一直存在争议。在这里，我们详细地重新评估了这个问题，并提供了强有力的证据表明 SUN4 是内核膜的组成部分，具有经典的 SUN 结构域蛋白拓扑结构。SUN4 的 C 端 SUN 结构域定位于核周空间，而 N 端则定向核质，与精子发生特异性的核纤层蛋白 B3 相互作用。我们发现 SUN4 在体内与 SUN3 形成异源二聚体组装，并调节 SUN3 的表达。总之，我们的研究结果有助于更好地理解 SUN4 在精子核-细胞质交界处的特定功能和精子头部形成的过程。
