Secondary Structural and Functional Studies of Rotavirus NSP4 and Caveolin-1 Peptide-Peptide Interactions

The rotavirus NSP4 protein is the first described viral enterotoxin. Abundant data from our laboratory reveals that NSP4 binds both the N- and C-termini of caveolin- 1 (aa2-31 and 161-178, respectively). Yeast two-hybrid and peptide binding analysis mapped the caveolin-1 binding site to three hydrophobic residues within the amphipathic a-helix, enterotoxic peptide domain (aa114-135). The research studies herein utilized peptides to investigate the interaction between NSP4 and caveolin-1. Peptides were synthesized corresponding to the amphipathic a-helix and caveolin-1 binding domain of NSP4 (aa112-140) and to the N- (aa2-20 and 19-40) and C- (161-178) termini of caveolin-1, and were utilized in structural and functional studies. Fluorescence binding assays revealed that NSP4 (aa112-140) binds to the N-terminus (aa19-40) of caveolin-1 with a stronger affinity than the C-terminus (aa161-178). In addition, this assay further delineated the NSP4 binding domain on caveolin-1 to aa19-40. Secondary structural changes following NSP4-caveolin-1 peptide-peptide interactions were investigated by circular dichroism analysis. Changes in a-helix formation were observed only upon interaction of the NSP4112-140 peptide with the C-terminal caveolin-1 peptide (C-Cav161- 178). The NSP4112-140 peptide contains a potential cholesterol recognition amino acid consensus (CRAC) sequence. Therefore this peptide was examined for cholesterol binding. Results of the binding assay revealed NSP4 binds cholesterol with a Kd of 7.67 +/- 1.49nM and this interaction occurs via aa112-140. Mutation of amino acid residues within the CRAC motif resulted in weaker binding affinities between each of the corresponding mutant peptides and cholesterol. NSP4 peptides containing mutations within the hydrophobic and charged faces of the amphipathic a-helix, enterotoxic peptide and caveolin-1 binding domain of NSP4 were examined for changes in secondary structure as well as diarrhea induction in mouse pups. Circular dichroism analysis revealed that mutation of hydrophobic residues resulted in a decrease in a-helix formation, whereas mutation of acidic and basic charged residues caused little to no change in a-helical content. When tested for diarrhea induction in mouse pups, the peptides containing mutations of either the hydrophobic or basic charged residues did not cause diarrhea. Taken together, the results of this research suggest a complex interplay between NSP4 secondary structure, caveolin-1 and cholesterol binding and diarrheagenic function.