Acts as an E3 ubiquitin-protein ligase specific for ldb1, mediating ubiquitination and proteasome-dependent degradation of excess ldb1 in a RING-dependent manner. Does not degrade ldb1 bound to lhx1/lim1, nor lim1 itself and thus contributes to the establishment of proper ldb1-lhx1/lim1 stoichiometry and the formation of a ldb1-lhx1/lim1 complex. Interferes with Spemann organizer function and suppresses secondary axis formation induced by ldb1 and lhx1/lim1. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Forms homodimers through the C-terminal region. The N-terminus interacts with the homeobox of LIM/homeobox factor lhx1/lim1, with lhx3/lim3 and lhx5/lim5, and with the N-terminus of ldb1 (By similarity). Belongs to the RNF12 family.