X-ray structure of engineered human Aortic Preferentially Expressed Protein-1 (APEG-1).
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 DeltaAPEG-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 DeltaAPEG-1 was determined and was refined to sub-atomic resolution (0.96 A). This is the best resolution for an immunoglobulin domain structure so far. The structure adopts a Greek-key beta-sandwich fold and belongs to the I (intermediate) set of the immunoglobulin superfamily. The residues lying between the beta-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 microM at physiological ionic strength, suggesting that APEG-1 dimerisation is only transient in the cell. The binding constant is strongly dependent on ionic strength.


CONCLUSIONS
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 physiologically relevant.
Manjasetty
Babu A
BA
Protein Structure Factory, c/o BESSY GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany. babu@bnl.gov
Niesen
Frank H
FH
Scheich
Christoph
C
Roske
Yvette
Y
Goetz
Frank
F
Behlke
Joachim
J
Sievert
Volker
V
Heinemann
Udo
U
Büssow
Konrad
K
D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001158:Arteries / Q000378:metabolism; D001703:Biophysics / Q000379:methods; D002448:Cell Adhesion; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D030562:Databases, Protein; D019281:Dimerization; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D007136:Immunoglobulins / Q000737:chemistry; D007700:Kinetics; D008239:Lysine / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000737:chemistry / Q000502:physiology*; D032389:Myocytes, Smooth Muscle / Q000378:metabolism; D009842:Oligopeptides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D015202:Protein Engineering; D017346:Protein Serine-Threonine Kinases; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D014461:Ultracentrifugation
D007136:Immunoglobulins; D009124:Muscle Proteins; D009842:Oligopeptides; C047981:arginyl-glycyl-aspartic acid; D017346:Protein Serine-Threonine Kinases; C100675:SPEG protein, human; D008239:Lysine