Efficient synthesis and comparative studies of the arginine and N^ω, N^ω-dimethylarginine forms of the human nucleolin glycine/arginine rich domain

The Gly- and Arg-rich C-terminal region of human nucleolin is a 61-residue long domain involved in a number of protein-protein and protein-nucleic acid interactions. This domain contains 10 aDma residues in the form of aDma-GG repeats interspersed with Phe residues. The exact role of Arg dimethylation is not known, partly because of the lack of efficient synthetic methods. This work describes an effective synthetic strategy, generally applicable to long RGG peptides, based on side-chain protected aDma and backbone protected dipeptide Fmoc-Gly-(Dmob)Gly-OH. This strategy allowed us to synthesize both the unmodified (N61Arg) and the dimethylated (N61aDma) peptides with high yield (∼26%) and purity. As detected by NMR spectroscopy, N61Arg does not possess any stable secondary or tertiary structure in solution and N^ω,N^ω-dimethylation of the guanidino group does not alter the overall conformational propensity of this peptide. While both peptides bind single-stranded nucleic acids with similar affinities (K_d = 1.5 × 10^-7 M), they exhibit a different behaviour in ssDNA affinity chromatography consistent with the difference in pK_a values. It has been previously shown that N61Arg inhibits HIV infection at the stage of HIV attachment to cells. This study demonstrates that Arg-dimethylated C-terminal domain lacks any inhibition activity, raising the question of whether nucleolin expressed on the cell-surface is indeed dimethylated.