Retrocyclin, an Antiretroviral θ-Defensin, Is a Lectin

Abstract

θ-Defensins are circular octadecapeptides that contain an internal tridisulfide ladder. Because retrocyclin-1, an ancestral hominid θ-defensin, can protect human cells in vitro from infection by T- and M-tropic strains of HIV-1, we used surface plasmon resonance techniques to study its binding to glycoproteins and glycolipids implicated in HIV-1 entry. Retrocyclin-1 bound with high affinity to gp120 (Kd, 35.4 nM), CD4 (Kd, 31 nM), and galactosylceramide (Kd, 24.1 nM). Neither a circular form of retrocyclin without its tridisulfide ladder nor its β-hairpin precursor with these disulfides intact bound gp120 or CD4 effectively. Retrocyclin also bound fetuin, an extensively glycosylated protein, with high affinity, but it did not bind nonglycosylated gp120 or BSA. However, retrocyclin did bind to a neoglycoprotein, BSA, with covalently attached sugar residues. Experiments with glycosidase-treated fetuin, gp120, and CD4 revealed that both O-linked and N-linked sugars were used as binding sites. In a panel of retrocyclin variants, binding to immobilized gp120 and CD4 were highly correlated to each other and to the peptide’s ability to protect human PBMCs from infection by HIV-1. Although small, cysteine-rich antimicrobial peptides with lectin-like properties exist in plants, θ-defensins are the first such molecules to be identified in vertebrates. Retrocyclin’s ability to recognize and bind carbohydrate-containing surface molecules is integrally related to its ability to protect cells from HIV-1 infection.

Energetics of Pore Formation Induced by Membrane Active Peptides

“…In experiments with model membranes, the peptide concentration P/L tends to be uniform over the entire membrane, but in activity assays on cells, a high local peptide concentration on the cell membrane can determine the fate of the cell. The recent discovery of high-affinity binding of θ-defensins to glycoproteins and glycolipids (57) points to a possibility of local accumulation of peptides on the membrane surface. Antimicrobial peptides clearly work by high concentration.…”

Trp-26 Imparts Functional Versatility to Human α-Defensin HNP1

“…The lectinlike properties of HNP1 were first demonstrated by its avid binding of glycoproteins such as HIV-1 gp120, a heavily glycosylated protein (56). Trp residues in small lectins have been shown to be important for oligosaccharide binding (57).…”

Functional Determinants of Human Enteric α-Defensin HD5

“…These small, 2-5-kDa peptides are classified into ␣, ␤, and families based on sequence homology and the connectivity of disulfide bonds linking the six conserved cysteine residues (6,7). Defensins bind carbohydrates (8,9), lipids (10,11), and DNA (12) and are active against a wide range of microorganisms, including bacteria (13,14) and viruses (15)(16)(17). Defensins are also capable of interacting with an diverse array of cellular receptors and host proteins, playing important immunomodulatory functions (5,18).…”