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Proinsulin C-peptide activates -enolase: implications for C-peptide–cell membrane interaction

Ishii, T;Fukano, K;Shimada, K;Kamikawa, A;Okamatsu-Ogura, Y;Terao, A;Yoshida, T;Saito, M;Kimura, K;

Proinsulin C-peptide shows beneficial effects on microvascular complications of Type 1 diabetes. However, the possible occurrence of membrane C-peptide receptor(s) has not been elucidated. The aim of this study was to identify and characterize membrane proteins to which C-peptide binds. The enzyme -enolase was co-immunoprecipitated with C-peptide after chemical cross-linking to HL-60 cell surface proteins and identified by mass spectrometry. Recombinant -enolase activity was modulated by C-peptide, with a significant decrease in K(m) for 2-phosphoglycerate without affecting V(max). The enzyme modulation by C-peptide was abolished when C-terminal basic lysine residue (K434) of the enzyme was replaced by neutral alanine or acidic glutamate, but not with basic arginine. The enzyme modulation by C-peptide was reproduced with the C-peptide fragments containing glutamate corresponding to position 27 (E27) of the full-length C-peptide. Addition of a lysine analogue to the assay and A31 cell culture abrogated the enzyme modulation and MAP kinase activation by C-peptide, respectively. The results indicate that C-peptide has the capacity to activate -enolase through a specific interaction between E27 of the peptide and K434 of the enzyme. Since -enolase plays a role as a cell surface receptor for plasminogen, it may conceivably also serve as a receptor for C-peptide in vivo.