Retrocyclin-2

Retrocyclins are circular mini-defensins with significant potential as agents against human immunodeficiency virus, influenza A, and herpes simplex virus. Retrocyclins bind carbohydrate-containing surface molecules such as gp120 and CD4 with high affinity (Kd, 10-100 nM), promoting their localization on cell membranes. The structural features important for activity have yet to be fully elucidated, but here, we have determined the first three-dimensional structure of a retrocyclin, namely, one of the most potent forms, retrocyclin-2. In the presence of SDS micelles, a well-defined beta-hairpin braced by three disulfide bonds that defines the cystine ladder motif is present. By contrast, a well-defined structure could not be determined in aqueous solution, suggesting that the presence of SDS micelles stabilizes the extended conformation of retrocyclin-2. Translational diffusion measurements indicate that retrocyclin-2 interacts with the SDS micelles, and such a membrane-like interaction may be an important feature in the mechanism of action of these antimicrobial peptides. Analytical ultracentrifugation and the NMR data indicated that retrocyclin-2 self-associates to form a trimer in a concentration-dependent manner. The ability to self-associate may contribute to the high-affinity binding of retrocyclins for glycoproteins by increasing the valency and enhancing the ability of retrocyclins to cross-link cell surface glycoproteins.

We tested the ability of retrocyclin 2, a type of theta defensin, to protect cells and chicken embryos from infection by H5N1 highly pathogenic avian influenza virus. A gene fragment of retrocyclin 2 was designed based on the protein sequence of retrocyclin 2 and cloned into the eukaryotic expression vector pcDNA4.01 (HismaxA), named pcDNA4-RC2. The expression vector pcDNA4-RC2 protected MDCK cells and chicken embryos from infection by the H5N1 virus through inhibition of virus replication and viral mRNA transcription. Retrocyclin 2 is therefore effective in preventing H5N1 virus infection in vivo and in vitro and could be considered as a new therapy for H5N1 influenza and other diseases.