The activity of cullin-RING type ubiquitination E3 ligases is regulated by 
neddylation, a process analogous to ubiquitination that culminates in covalent 
attachment of the ubiquitin-like protein Nedd8 to cullins. As a component of the 
E3 for neddylation, SCCRO/DCUN1D1 plays a key regulatory role in neddylation 
and, consequently, cullin-RING ligase activity. The essential contribution of 
SCCRO to neddylation is to promote nuclear translocation of the cullin-ROC1 
complex. The presence of a myristoyl sequence in SCCRO3, one of four SCCRO 
paralogues present in humans that localizes to the membrane, raises questions 
about its function in neddylation. We found that although SCCRO3 binds to CAND1, 
cullins, and ROC1, it does not efficiently bind to Ubc12, promote cullin 
neddylation, or conform to the reaction processivity paradigms, suggesting that 
SCCRO3 does not have E3 activity. Expression of SCCRO3 inhibits SCCRO-promoted 
neddylation by sequestering cullins to the membrane, thereby blocking its 
nuclear translocation. Moreover, SCCRO3 inhibits SCCRO transforming activity. 
The inhibitory effects of SCCRO3 on SCCRO-promoted neddylation and 
transformation require both an intact myristoyl sequence and PONY domain, 
confirming that membrane localization and binding to cullins are required for in 
vivo functions. Taken together, our findings suggest that SCCRO3 functions as a 
tumor suppressor by antagonizing the neddylation activity of SCCRO.