Characterization of the Secondary Structure of CP30, a Highly Repetitive Ampholytic Protein in Beetle Elytral Cuticle

Abstract

Summary: CP30 is a major component protein in the rigid cuticle of the beetle elytron which we recently identified. It has a highly unusual amino acid sequence comprised largely of alternating blocks of 3-5 anionic and cationic amino acid residues (three charged residues, glutamic acid, arginine and histidine, make up 76% of the sequence) which suggest an unusual stuctural role. The secondary structure of CP30 was evaluated at a variety of pH, ionic strength and temperature conditions. Circular dichroism (CD), fluorescence spectroscopy and static and dynamic light scattering showed that the protein had an extended a-helical structure at pH 3, 5 and 7 which unfolded around 60 C and reformed upon cooling back to 10 C. The protein aggregated or adhered to the dialysis membrane at pH 5, low salt, which is close to the protein's isoelectric point (5.82), preventing analysis at that condition. Further studies aimed at solubilizing the protein near that condition may uncover evidence of assembly. While the structural function of CP30 is currently unknown, the sequence of CP30 may offer new design motifs for the development of novel polymer or gel biomaterials.