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

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.

Mouse Odf2 localizes to centrosomes and basal bodies in adult tissues and to the photoreceptor primary cilium.

Odf2 (outer dense fiber 2) is the major protein of the cytoskeleton of the sperm tail. In somatic cells, it is a component of the centrosome in which it is located in the appendages of the mother centriole. Additionally, as shown previously by forced expression in cultured cells, Odf2 localizes to centrioles, basal bodies, and primary cilia, which are all structurally and functionally interconnected.