The crystal structure of CREG, a secreted glycoprotein involved in cellular growth and differentiation.
The cellular repressor of E1A-stimulated genes (CREG) is a secreted glycoprotein that inhibits proliferation and enhances differentiation of human embryonal carcinoma cells. CREG binds to the cation-independent mannose 6-phosphate (M6P)/insulin-like growth factor II (IGF2) receptor (IGF2R) (M6P/IGF2R), and this receptor has been shown to be required for CREG-induced growth suppression. To better understand CREG function in cellular growth and differentiation, we solved the 3D crystal structure of this protein to 1.9-A resolution. CREG forms a tight homodimeric complex, and CREG monomers display a beta-barrel fold. The three potential glycosylation sites on CREG map to a confined patch opposite the dimer interface. Thus, dimerization of glycosylated CREG likely presents a bivalent ligand for the M6P/IGF2R. Closely related structural homologs of CREG are FMN-binding split-barrel fold proteins that bind flavin mononucleotide. Our structure shows that the putative flavin mononucleotide-binding pocket in CREG is sterically blocked by a loop and several key bulky residues. A mutant of CREG lacking a part of this loop maintained overall structure and dimerization, as well as M6P/IGF2R binding, but lost the growth suppression activity of WT CREG. Thus, analysis of a structure-based mutant of CREG revealed that binding to M6P/IGF2R, while necessary, is not sufficient for CREG-induced growth suppression. These findings indicate that CREG utilizes a known fold for a previously undescribed function [corrected]
Sacher
Michael
M
Montreal Proteomics Network, 740 Doctor Penfield, Montreal, QC, Canada H3A 1A4. michael.sacher@bri.nrc.ca
Di Bacco
Alessandra
A
Lunin
Vladimir V
VV
Ye
Zheng
Z
Wagner
John
J
Gill
Grace
G
Cygler
Miroslaw
M
D000595:Amino Acid Sequence; D000818:Animals; D002454:Cell Differentiation; D049109:Cell Proliferation; D018360:Crystallography, X-Ray; D019281:Dimerization; D006023:Glycoproteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D017527:Receptor, IGF Type 2 / Q000378:metabolism; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D040681:Structural Homology, Protein
C114068:CREG1 protein, human; D006023:Glycoproteins; D017527:Receptor, IGF Type 2; D012097:Repressor Proteins