Research article
X-ray structure of engineered human Aortic Preferentially Expressed Protein-1 (APEG-1)
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* Corresponding authors: Udo Heinemann heinemann@mdc-berlin.de - Konrad Büssow buessow@molgen.mpg.de
1 Protein Structure Factory, c/o BESSY GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany
2 Max-Delbrück-Centrum für Molekulare Medizin, Robert-Rössle-Str. 10, 13092 Berlin, Germany
3 Institut für Chemie/Kristallographie, Freie Universität, Takustr. 6, 14195 Berlin, Germany
4 Protein Structure Factory, Heubnerweg 6, 14059 Berlin, Germany
5 Max-Planck-Institut für Molekulare Genetik, Ihnestr. 73, 14195 Berlin, Germany
6 Charité Universitätsmedizin Berlin, Institut für Medizinische Physik & Biophysik, Ziegelstr. 5-9, 10098 Berlin, Germany
7 Structural Genomics Consortium, University of Oxford, Botnar Research Centre, Oxford, OX3 7LD, UK
8 Case Centre for Proteomics, Case Western Reserve University, Upton, New York 11973, USA
BMC Structural Biology 2005, 5:21 doi:10.1186/1472-6807-5-21
Published: 14 December 2005Abstract
Background
Human Aortic Preferentially Expressed Protein-1 (APEG-1) is a novel specific smooth muscle differentiation marker thought to play a role in the growth and differentiation of arterial smooth muscle cells (SMCs).
Results
Good quality crystals that were suitable for X-ray crystallographic studies were obtained following the truncation of the 14 N-terminal amino acids of APEG-1, a region predicted to be disordered. The truncated protein (termed ΔAPEG-1) consists of a single immunoglobulin (Ig) like domain which includes an Arg-Gly-Asp (RGD) adhesion recognition motif. The RGD motif is crucial for the interaction of extracellular proteins and plays a role in cell adhesion. The X-ray structure of ΔAPEG-1 was determined and was refined to sub-atomic resolution (0.96 Å). This is the best resolution for an immunoglobulin domain structure so far. The structure adopts a Greek-key β-sandwich fold and belongs to the I (intermediate) set of the immunoglobulin superfamily. The residues lying between the β-sheets form a hydrophobic core. The RGD motif folds into a 310 helix that is involved in the formation of a homodimer in the crystal which is mainly stabilized by salt bridges. Analytical ultracentrifugation studies revealed a moderate dissociation constant of 20 μM at physiological ionic strength, suggesting that APEG-1 dimerisation is only transient in the cell. The binding constant is strongly dependent on ionic strength.
Conclusion
Our data suggests that the RGD motif might play a role not only in the adhesion of extracellular proteins but also in intracellular protein-protein interactions. However, it remains to be established whether the rather weak dimerisation of APEG-1 involving this motif is physiogically relevant.