SPSB proteins regulate adipocyte differentiation by targeting FOG-2 for proteasomal degradation.

Friend of GATA (FOG)-2 is a transcriptional cofactor that cooperates with GATA family transcription factors to regulate the development of multiple organs and tissues. During adipocyte differentiation, FOG-2 protein is rapidly downregulated, and this reduction is thought to be important for efficient differentiation, but the underlying mechanism has not been elucidated. Here, we show that the ubiquitin-proteasome system degrades FOG-2 during adipogenesis and that SPRY domain- and SOCS box-containing (SPSB) proteins, the substrate-recognition subunits of Cullin-RING ligase 5 (CRL5), mediate this process. A bioinformatic screen based on the consensus D/E-I/L-N-N-N motif and the biochemical properties of SPSB1, SPSB2, and SPSB4 (SPSB1/2/4) identified FOG-2 as a candidate substrate. We found that SPSB1/2/4 bind FOG-2 by recognizing a D-L-N-N-N sequence at residues 134-138 in its N-terminal region, with SPSB4 most efficiently promoting its ubiquitin-proteasome-dependent degradation. Interestingly, SPSB3, which employs a substrate-recognition mechanism distinct from that of SPSB1/2/4, also binds FOG-2 independently of this N-terminal motif but fails to trigger its degradation. Finally, in 3T3-L1 preadipocytes, inhibition of SPSB1/2/4-dependent FOG-2 degradation increases FOG-2 protein levels and suppresses adipocyte differentiation, indicating that timely removal of FOG-2 is functionally important in this context. Together, these findings identify FOG-2 as an SPSB-regulated substrate and support a role for the SPSB1/2/4-FOG-2 axis as a previously unrecognized mechanism that regulates adipocyte differentiation.