High affinity binding of hydrophobic and autoantigenic regions of proinsulin to the 70 kDa chaperone DnaK.
BACKGROUND
Chaperones facilitate proper folding of peptides and bind to misfolded proteins as occurring during periods of cell stress. Complexes of peptides with chaperones induce peptide-directed immunity. Here we analyzed the interaction of (pre)proinsulin with the best characterized chaperone of the hsp70 family, bacterial DnaK.


RESULTS
Of a set of overlapping 13-mer peptides of human preproinsulin high affinity binding to DnaK was found for the signal peptide and one further region in each proinsulin domain (A- and B-chain, C-peptide). Among the latter, peptides covering most of the B-chain region B11-23 exhibited strongest binding, which was in the range of known high-affinity DnaK ligands, dissociation equilibrium constant (K'd) of 2.2 ± 0.4 μM. The B-chain region B11-23 is located at the interface between two insulin molecules and not accessible in insulin oligomers. Indeed, native insulin oligomers showed very low DnaK affinity (K'd 67.8 ± 20.8 μM) whereas a proinsulin molecule modified to prevent oligomerization showed good binding affinity (K'd 11.3 ± 7.8 μM).


CONCLUSIONS
Intact insulin only weakly interacts with the hsp70 chaperone DnaK whereas monomeric proinsulin and peptides from 3 distinct proinsulin regions show substantial chaperone binding. Strongest binding was seen for the B-chain peptide B 11-23. Interestingly, peptide B11-23 represents a dominant autoantigen in type 1 diabetes.
German Diabetes Centre, Leibniz Institute at Heinrich Heine University Düsseldorf, Institute of Clinical Diabetology, Auf'm Hennekamp 65, D-40225 Düsseldorf, Germany. burkart@ddz.uni-duesseldorf.de
Volker
V
Burkart
Rahel K
RK
Siegenthaler
Elias
E
Blasius
Koen
K
Vandenbroeck
Iraide
I
Alloza
Waltraud
W
Fingberg
Nanette C
NC
Schloot
Philipp
P
Christen
Hubert
H
Kolb
D000595:Amino Acid Sequence; D001324:Autoantigens / Q000276:immunology / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism; D017124:Conserved Sequence; D019143:Evolution, Molecular; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D007328:Insulin / Q000378:metabolism; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011384:Proinsulin / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D011485:Protein Binding; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary
D001324:Autoantigens; D001426:Bacterial Proteins; D018840:HSP70 Heat-Shock Proteins; D007328:Insulin; D010446:Peptide Fragments; D011384:Proinsulin