SSX2IP helps the centrosome prepare for mitosis by delivering essential proteins, Bärenz et al. report.
Constructing the mitotic spindle is a logistical challenge. Before mitosis begins, the cell has to synthesize and position a host of proteins, including centrosome proteins that arrange microtubules at the spindle poles and control their growth. One of these arrivals is the γ-tubulin ring complex (γ-TuRC), which gathers in the pericentriolar material of the centrosome to spur microtubule nucleation, but additional microtubule-binding proteins are required for spindle assembly.
Bärenz et al. tallied the microtubule-binding proteins during interphase and metaphase in unfertilized Xenopus eggs. One protein, synovial sarcoma X breakpoint 2 interacting protein (SSX2IP), stood out because researchers hadn’t previously identified it as a spindle protein. However, SSX2IP was essential for centrosome maturation. In Xenopus egg extracts, for example, removal of SSX2IP hindered γ-TuRC accumulation at the centrosome, reducing microtubule nucleation and hampering spindle assembly. The pericentriolar material broke apart and mitotic progression slowed in human somatic cells with reduced SSX2IP levels.
SSX2IP promotes the assembly of a functional centrosome by ferrying γ-TuRC and other proteins. In human somatic cells, SSX2IP rides in centriolar satellites, which carry pericentriolar material to the centrosome. SSX2IP shows an unusual pattern of expression. A cell typically synthesizes spindle proteins before mitosis and destroys them afterward. But SSX2IP remains throughout interphase, suggesting that the organism sets the levels of the protein only once in its lifetime, during egg maturation.
Text by Mitch Leslie