Mihwa Lee

Implementing the LIM code: the structural basis for cell type-specific assembly of LIM-homeodomain complexes

LIM‐homeodomain (LIM‐HD) transcription factors form a combinatorial ‘LIM code’ that contributes to the specification of cell types. In the ventral spinal cord, the binary LIM homeobox protein 3 (Lhx3)/LIM domain‐binding protein 1 (Ldb1) complex specifies the formation of V2 interneurons. The additional expression of islet‐1 (Isl1) in adjacent cells instead specifies the formation of motor neurons through assembly of a ternary complex in which Isl1 contacts both Lhx3 and Ldb1, displacing Lhx3 as the binding partner of Ldb1. However, little is known about how this molecular switch occurs. Here, we have identified the 30‐residue Lhx3‐binding domain on Isl1 (Isl1LBD). Although the LIM interaction domain of Ldb1 (Ldb1LID) and Isl1LBD share low levels of sequence homology, X‐ray and NMR structures reveal that they bind Lhx3 in an identical manner, that is, Isl1LBD mimics Ldb1LID. These data provide a structural basis for the formation of cell type‐specific protein–protein interactions in which unstructured linear motifs with diverse sequences compete to bind protein partners. The resulting alternate protein complexes can target different genes to regulate key biological events.

An orthorhombic form of Escherichia coli aminopeptidase P at 2.4 A resolution

Aminopeptidase P (AMPP) from Escherichia coli cleaves the N-terminal residue from an oligopeptide if the second residue is proline. The active site contains a dinuclear metal centre. Following earlier structural analyses of crystals in space groups P6(4)22 and I4(1)22, the structure of AMPP has been solved and refined in the orthorhombic space group C222(1) at 2.4 A resolution. There are six subunits in the asymmetric unit. These are arranged in two types of tetramer. One tetramer comprises four crystallographically independent subunits, while the other comprises two pairs of subunits related by a crystallographic twofold axis. The final model of 20 994 protein atoms, 1618 water molecules and 12 metal atoms refined to residuals R = 0.195 and R(free) = 0.215. The molecular structure confirms most of the previously reported features, including the subunit-subunit interfaces in the tetramer and persistent disorder at some residues. The metal-ligand bond lengths at the active site suggest that one of the two Mn atoms is five-coordinate rather than six-coordinate.

Crystallization of an Lhx3-Isl1 complex.

A stable intramolecular complex comprising the LIM domains of the LIM-homeodomain protein Lhx3 tethered to a peptide region of Isl1 has been engineered, purified and crystallized. The monoclinic crystals belong to space group C2, with unit-cell parameters a = 119, b = 62.2, c = 51.9 A, beta = 91.6 degrees , and diffract to 2.05 A resolution.

Crystallization of a ZRANB2-RNA complex

ZRANB2 is a zinc-finger protein that has been shown to influence alternative splice-site selection. The protein comprises a C-terminal arginine/serine-rich domain that interacts with spliceosomal proteins and two N-terminal RanBP2-type zinc fingers that have been implicated in RNA recognition. The second zinc finger bound to a six-nucleotide single-stranded RNA target sequence crystallized in the hexagonal space group P6(5)22 or P6(1)22, with unit-cell parameters a = 54.52, b = 54.52, c = 48.07 A; the crystal contains one monomeric complex per asymmetric unit. This crystal form has a solvent content of 39% and diffracted to 1.4 A resolution using synchrotron radiation.

Crystallization of Pichia pastoris lysyl oxidase

A copper-containing amine oxidase (PPLO) from the yeast Pichia pastoris has been purified and crystallized in two forms. PPLO is a glycoprotein. The molecular mass from SDS-polyacrylamide gels is 112 kDa, consistent with 20% glycosylation by weight (the calculated molecular weight of the polypeptide is 89.7 kDa). Orthorhombic crystals belonging to space group P2(1)2(1)2(1), with unit-cell parameters a = 163.7, b = 316.1, c = 84.0 A, diffract to 2.65 A resolution. Monoclinic crystals belonging to space group C2, with unit-cell parameters a = 248.4, b = 121.1, c = 151.8 A, beta = 124.6 degrees, diffract to 1.65 A resolution. Native data have been recorded from each crystal form at 100 K using synchrotron radiation. A self-rotation function for the monoclinic crystal form reveals the presence of a non-crystallographic twofold axis perpendicular to the crystallographic twofold axis, consistent with the presence of two dimers in the asymmetric unit.