The Spin/Ssty repeat: a new motif identified in proteins involved in vertebrate development from gamete to embryo

The homologous genes Spin (spindlin) and Ssty were first identified as genes involved in gametogenesis and seem to occur in multiple copies in vertebrate genomes. The mouse spindlin (Spin) protein was reported to interact with the spindle apparatus during oogenesis and to be a target for cell-cycle-dependent phosphorylation. The transcript of the mouse Ssty gene is specific to sperm cells. In the chicken, spindlin was found to co-localize with SUMO-1 to nuclear dots during interphase in fibroblasts, but to co-localize with chromosomes during mitosis. Thus, Spin/Ssty genes might be important in the transition from sperm cells and oocytes to the early embryo, as well as in mitosis.

Here we report the discovery of a new protein motif of around 50 amino acids in length, the Spin/Ssty repeat, in proteins of the Spin/Ssty (spindlin) family. We found that in one member of this family, the human SPIN gene, each repeat resides in its own exon, supporting our view that Spin/Ssty repeats are independent functional units. On the basis of different secondary-structure prediction methods, we propose a four-stranded beta-structure for the Spin/Ssty repeat.

The discovery of the Spin/Ssty repeat might contribute to the further elucidation of the structure and function of spindlin-family proteins. We predict that the tertiary structure of spindlin-like proteins is composed of three modules of Spin/Ssty repeats.