SPAG4L/SPAG4L-2 are testis-specific SUN domain proteins restricted to the apical nuclear envelope of round spermatids facing the acrosome

Abstract

SUN domain proteins are integral proteins of the inner nuclear membrane and functions in linkage of the nuclear lamina to the cytoskeleton. Moreover, SUN domain proteins seem to mediate the tethering of the centrosome to the nuclear membrane, and they are involved in telomere attachment to the nuclear envelope in meiotic cells, and in germ cell development in invertebrates. In contrast to the widely expressed SUN domain proteins in mammals, SUN1 and SUN2, which have been analysed in great detail, there is virtually nothing known about testicular SUN domain proteins. Since a hallmark of male germ cell development is the profound remodelling of the nuclear envelope, emphasized, for example, by the reshaping of the nucleus during spermiogenesis, and the biogenesis of its tightly associated acrosome, SUN domain proteins might be engaged in these processes. We have isolated a novel SUN domain protein, SPAG4L-2, that differs from SPAG4L by an N-terminal insertion of 25 amino acids. Spag4l and Spag4l-2 are exclusively expressed in testis at about equimolar amounts, and show elevated transcription during ongoing spermiogenesis coincident with the appearance of round spermatids. Molecular dissection of the protein followed by cytological and biochemical investigations revealed that SPAG4L-2 and SPAG4L are transmembrane proteins that localize to the nuclear envelope. SPAG4L/4L-2 are restricted to the apical nuclear region of round spermatids that face the acrosomic vesicle, and thus are most probably involved in linkage of the acrosomic vesicle to the spermatid nucleus, and in acrosome biogenesis.

Absence of Dpy19l2, a new inner nuclear membrane protein, causes globozoospermia in mice by preventing the anchoring of the acrosome to the nucleus

“…In this work, we have characterized a new family of proteins located in the INM, the absence of which leads to the disruption of the bridging structure required for acrosome attachment on the nucleus. Interestingly, Sun5, recently described in the NE facing the acrosome in mouse, shows a spatiotemporal pattern of expression very similar to that of Dpy19l2 (Frohnert et al, 2011). Our results show that SUN5 and DPY19L2 clustered in the NE in transfected HEK cells and partially colocalized.…”

“…DPY19L2 was tagged with GFP, and DPY19L3 and SUN5 were tagged with DDK. We chose SUN5 as a marker of the inner nuclear membrane (INM) because it was recently demonstrated that SUN5 is positioned in round spermatids in the NE facing the acrosomal vesicle (Frohnert et al, 2011).…”

Involvement of Classical Bipartite/Karyopherin Nuclear Import Pathway Components in Acrosomal Trafficking and Assembly During Bovine and Murid Spermiogenesis1

“…In this context consideration should be given to a novel testis-specific SUN domain protein (SPAG4L-2), a transmembrane protein restricted to the apical nuclear region of round spermatids that faces the AV, as a possible component of a protein linkage complex that anchors the AV to the nuclear membrane [67]. It is well documented that SUN and KASH domain proteins, spanning the nuclear envelope, organize a diversity of nucleo-cytoplasmic connections, including tethering microtubules and centromeres to the nuclear envelope [68].…”

SUN4 is essential for nuclear remodeling during mammalian spermiogenesis

“…Male germ-line cells contain three other SUN proteins (Kracklauer et al, 2013), SUN3 (Göb et al, 2010), SUN4 (originally termed Spag4 (Shao et al, 1999;Tarnasky et al, 1998)) and SUN5 (also known as Spag4-like (Frohnert et al, 2011;Jiang et al, 2011)). Each of these conforms to the same general domain organization as SUN1/2 (Hasan et al, 2006;Liu et al, 2007).…”