MEARA sequence repeat of human CstF-64 polyadenylation factor is helical in solution. A spectroscopic and calorimetric study

The primary structure of the human CstF-64 polyadenylation factor contains 12 nearly identical repeats of a consensus motif of five amino acid residues with the sequence MEAR(A/G). No known function has yet been ascribed to this motif; however, according to secondary structure prediction algorithms, it should form a helical structure in solution. To validate this theoretical prediction, we synthesized a 31 amino acid residue peptide (MEARA₆) containing six repeats of the MEARA sequence and characterized its structure and stability by circular dichroism (CD) spectroscopy and differential scanning calorimetry (DSC). No effects of concentration on the CD or DSC properties of MEARA₆ were observed, indicating that the peptide is monomeric in solution at concentrations up to 2 mM. The far UV-CD spectra of MEARA₆ indicates that at a low temperature (1 °C) the MEARA₆ peptide has a relatively high helical content (76% at pH 2.0 and 65% at pH 7.0). The effects of pH and ionic strength on the CD spectrum of MEARA₆ suggest that a number of electrostatic interactions (e.g., i, i + 3 Arg/Glu ion pair, charge-dipole interactions) contribute to the stability of the helical structure in this peptide. DSC profiles show that the melting of MEARA₆ helix is accompanied by positive change in the enthalpy. To determine thermodynamic parameters of helix-coil transition from DSC profiles for this peptide, we developed a new, semiempirical procedure based on the calculated function for the heat capacity of the coiled state for a broad temperature range. The application of this approach to the partial molar heat capacity function for MEARA₆ provides the enthalpy change for helix formation calculated per amino acid residue as 3.5 kJ/mol.