Mutations to the protein SPOP are widespread in cancer, yet many remain poorly understood. To address this gap, St. Jude Children's Research Hospital scientists obtained structures of SPOP in both the presence and absence of these mutations, according to a July 13 announcement. Their work captured the fine balance between active and inactive states of SPOP, showing how its activity is regulated and revealing that a key subset of cancer mutations disrupt this balance. The study was published in Molecular Cell.
SPOP acts as a substrate receptor within the E3 ubiquitin ligase complex that balances levels of specific proteins in cells, including gene-regulators BRD2, BRD3 and BRD4. These regulators contribute to cancer development when their activity or levels are dysregulated. While many SPOP cancer mutations affect how these proteins bind to SPOP—making their role in cancer clear—mutations elsewhere on SPOP have remained an open question.
The research team led by Tanja Mittag found that individual SPOP molecules assemble into a large ring-like "double-donut" state when inactive. When activated by its binding partner Cullin-3, it forms a linear "filament" state instead. Gain-of-function mutations exclusively form filaments while loss-of-function mutations favor the double-donut formation, tipping the equilibrium abnormally toward either inactivity or overactivity.
“The double-donut assembly is made up of two stacked rings, formed by between 22 and 30 individual SPOP molecules, and is wide enough to encircle an entire ribosome,” Mittag said. Previous studies showed how long filaments facilitate substrate binding; these new structures show Cullin-3 activates SPOP by driving filament assembly.
“One of the most important roles of the double donut is that it represents an 'off' state; they are essentially an autoinhibited, inactive form of SPOP. The linear filament form is the active state,” said co-first author Matt Cuneo. “The cancer mutations can bypass this regulation, meaning they are no longer responding to normal cellular signals that switch SPOP between the inactive double-donut structure and the active filament.”
These findings have potential therapeutic implications for targeting abnormal forms of SPOP in cancer treatment strategies.
