THE LIM homeodomain (LIM-HD) proteins, which contain two tandem LIM do
mains followed by a homeodomain, are critical transcriptional regulato
rs of embryonic development(1-5). The LIM domain is a conserved cystei
ne-rich zinc-binding motif found in LIM-HD and LMO (rhombotin or Ttg)
proteins, cytoskeletal components, LIM kinases and other proteins(1).
LIM domains are protein-protein interaction motifs(1), can binding of
LIM-HD proteins to DNA(6,7) and can negatively regulate LIM-HD protein
function(8). How LIM domains exert these regulatory effects is not kn
own. We have now isolated a new LIM-domain-binding factor, LdB1, on th
e basis of tis ability to interact with the LIM-HD protein Lhx1 (Lim1)
(9). High-affinity binding by Ldb1 requires paired LIM domains and is
restricted to the related subgroup of LIM domains found in LIM-HD and
LMO proteins. The highly conserved Xenopus Lbd protein XLbd1, interact
s with Xlim-1, the Xenopus orthologue of Lhx1. When injected into Xeno
pus embryos, XLbd1 (or Lbd1) can synergize with Xlim-1 in the formatio
n of partial secondary axes and in activation of the genes encoding go
osecoid (gsc), chordin, NCAM and XCG7, demonstrating a functional as w
ell as a physical interaction between the two proteins.