Fig. 10. A model for the role of XRnf12 in the establishment of proper Xlim-1/Ldb1 stoichiometry in the Spemann organizer. In the organizer, tetramer formation of Xlim-1/Ldb1 is required for their activity. XRnf12 selectively degrades excess Ldb1 unbound to Xlim-1, which interferes with organizer gene expression presumably by disturbing Xlim-1/Ldb1 tetramer formation. Excess Ldb1 may also possibly interfere with LIM domain-dependent association of Xlim-1 with other proteins. In this way, proper stoichiometry and maximal activity of Xlim-1/Ldb1 is assured in the presence of XRnf12 in the organizer. In the ventrolateral mesoderm, Ldb1 may escape degradation by XRnf12 through interaction with Ldb1-interacting proteins, one of which may be XLMO4 (J. L. Gomez-Skarmeta, personal communication). The putative Ldb1/LMO complex may contribute to complete suppression of Xlim-1/Ldb1 activity in the ventrolateral region, and may participate in a distinct transcriptional regulatory complex. Xlim-1 unbound to Ldb1 may be subject to proteasome-dependent degradation by an unidentified ubiquitin ligase, similar to the case of Drosophila Apterous (Weihe et al., 2001).