Kinetochore formation involves some SUMO wrestling. Two opposing proteins involved in the SUMO pathway control assembly of the structures, Mukhopadhyay et al. reveal.
Addition of a SUMO molecule can alter a protein's location, stability, and other characteristics. But SUMOylation can also lead to the protein's demise. The enzyme RNF4 targets proteins carrying SUMO chains of a certain length, tagging them with ubiquitin to spur their destruction. Another enzyme, SENP6, spares proteins by shortening SUMO chains. Mukhopadhyay et al. investigated how the balance between SENP6 and RNF4 affects kinetochore assembly.
Mitosis stalled in cells lacking SENP6, and chromosomes often didn't line up properly during metaphase. Those defects also appear in cells that are missing the CENP-H/I/K complex, which helps insert other molecules into the forming kinetochore. When SENP6 was absent, CENP-H and CENP-I vanished from the inner part of the kinetochores. So did other proteins that the CENP-H/I/K complex helps put in place, such as CENP-O.
Further experiments suggested that the missing CENP-I had been destroyed. Removing RNF4 or disabling the proteasome, which demolishes ubiquitin-tagged molecules, caused CENP-I levels to return to normal. The overall picture is that SENP6 permits kinetochore assembly by preventing RNF4 from ubiquitinating CENP-I. Why cells adopt this indirect mechanism to control ubiquitination isn't clear. CENP-I might require SUMOylation to do its job, the researchers speculate.
