Structural changes of human RNase L upon homodimerization investigated by Raman spectroscopy.

RNase L, a key enzyme in the host defense system, is activated by the binding of 2′–5′-linked oligoadenylates (2–5A) to the N-terminal ankyrin repeat domain, which causes the inactive monomer to form a catalytically active homodimer. We focused on the structural changes of human RNase L as a result of interactions with four different activators: natural 2–5 pA 4 and three tetramers with 3′-end AMP units replaced with ribo- , arabino- and xylo- configured phosphonate analogs of AMP (pA 3 X). The extent of the RNase L dimerization and its cleavage activity upon binding of all these activators were similar. A drop-coating deposition Raman (DCDR) spectroscopy possessed uniform spectral changes upon binding of all of the tetramers, which verified the same binding mechanism. The estimated secondary structural composition of monomeric RNase L is 44% α-helix, 28% β-sheet, 17% β-turns and 11% of unordered structures, whereas dimerization causes a slight decrease in α-helix and increase in β-sheet (ca. 2%) content. The dimerization affects at least three Tyr, five Phe and two Trp residues. The α–β structural switch may fix domain positions in the hinge region (residues ca. 336–363) during homodimer formation. [ABSTRACT FROM AUTHOR]