A biologically active peptide mimetic of N-acetylgalactosamine/galactose

A 12-mer phage display library was screened with a GalNAc-specific lectin to identify an amino acid sequence that binds to the lectin. Phage particles that were eluted from the lectin with free GalNAc were considered to have been bound to a GalNAc-binding site. Peptides were synthesized with the selected sequence as a quadravalent structure to facilitate receptor crosslinking. Treatment of human peripheral blood mononuclear cells for 24 h with the peptide stimulated secretion of interleukin-8 (IL-8) but not of IL-1β, IL-6, IL-10, or tumor necrosis factor-α (TNF-α). The secretion of IL-21 was stimulated as strongly with the peptide as with interferon-γ.The data indicate that the quadravalent peptide has biological activity with a degree of specificity. These effects occurred at concentrations in the nanomolar range, in contrast to free sugars that generally bind to proteins in the micro- to millimolar range.Many cells express cell-surface receptors that bind sugar-containing ligands and serve important regulatory functions [1]. Extensive research over the past two decades has been devoted to design of peptide mimetics of sugars [2] to serve as vaccines that elicit anti-carbohydrate antibodies [3,4] or to bind with high affinity to specific antibodies [5,6]. We asked whether a peptide mimetic of N-acetylgalactosamine (GalNAc) could be identified that induces specific responses. For this purpose, a phage display library was screened with a GalNAc-specific lectin as a receptor analog. A consensus amino acid sequence emerged in the variable region of the pIII protein in the selected phage particles. Because clusters of GalNAc bind to receptors with higher affinity than a single residue [7], and receptor crosslinking is often required for many signal transduction mechanisms [8], we designed and tested a multivalent structure containing this sequence.The lectin from Helix pomatia (HPA) binds O-linked α-GalNAc but also recognizes Gal (β1–3) GalNAc and α-GlcNAc [9,10]. KA va