Equilibrium unfolding of an oligomeric protein involves formation of a multimeric intermediate state(s)

Characterization of the structural forces governing the reversibility of the thermal unfolding of the human acidic fibroblast growth factor

Human acidic fibroblast growth factor (hFGF1) is an all beta-sheet protein that is involved in the regulation of key cellular processes including cell proliferation and wound healing. hFGF1 is known to aggregate when subjected to thermal unfolding. In this study, we investigate the equilibrium unfolding of hFGF1 using a wide array of biophysical and biochemical techniques. Systematic analyses of the thermal and chemical denaturation data on hFGF1 variants (Q54P, K126N, R136E, K126N/R136E, Q54P/K126N, Q54P/R136E, and Q54P/K126N/R136E) indicate that nullification of charges in the heparin-binding pocket can significantly increase the stability of wtFGF1. Triple variant (Q54P/K126N/R136E) was found to be the most stable of all the hFGF1 variants studied. With the exception of triple variant, thermal unfolding of wtFGF1 and the other variants is irreversible. Thermally unfolded triple variant refolds completely to its biologically native conformation. Microsecond-level molecular dynamic simulations reveal that a network of hydrogen bonds and salt bridges linked to Q54P, K126N, and R136E mutations, are responsible for the high stability and reversibility of thermal unfolding of the triple variant. In our opinion, the findings of the study provide valuable clues for the rational design of a stable hFGF1 variant that exhibits potent wound healing properties.

A superior drug carrier--aponeocarzinostatin in partially unfolded state fully protects the labile antitumor enediyne

Background

Neocarzinostatin is a potent antitumor drug consisting of an enediyne chromophore and a protein carrier.

Methods

We characterized an intermediate in the equilibrium unfolding pathway of aponeocarzinostatin, using a variety of biophysical techniques including 1-anilino-8-napthalene sulfonate binding studies, size-exclusion fast protein liquid chromatography, intrinsic tryptophan fluorescence, circular dichroism, and ¹H-¹⁵N heteronuclear single quantum coherence spectroscopy.

Results

The partially unfolded protein is in molten globule-like state, in which ~60% and ~20% tertiary and secondary structure is disrupted respectively. Despite lacking a fully coordinated tertiary structure for assembling a functional binding cleft, the protein in molten globule-like state is still able to fully protect the labile chromophore. Titration of chromophore leads the partially denatured apoprotein to fold into its native state.

Conclusion

These findings bring insight into conserving mechanism of neocarzinostatin under harsh environment, where even the partially denatured apoprotein exhibits protective effect, confirming the superiority of the drug carrier.

Non-compact nucleocapsid protein multimers in influenza-virus-infected cells

We have previously shown that protease-resistant and highly immunoreactive compact NP oligomers, dissociating at +80 degrees C and possessing properties of folded proteins, are post-translationally formed in influenza-virus-infected cells. In this study we demonstrate that, in addition to compact NP oligomers, incompletely folded NP multimers are detected intracellularly by SDS/PAGE carried out under weak dissociating conditions. In cells infected with avian, human A(H2N2), and human A(H3N2) viruses, NP multimers are detected in the stacking gel of SDS/PAGE as retarded and loose structures dissociating at +50 degrees C. NP multimers are more sensitive to proteolysis than NP oligomers, but they are more resistant to proteolysis than NP monomers. In contrast to compact NP oligomers, NP multimers possess a weak immunoreactivity to some monoclonal antibodies. Pulse-chase experiments have shown that NP multimers appear at early stages of NP synthesis and are partially converted post-translationally into faster-migrating compact NP oligomers. In the course of infection, the excess NP multimers not converted into compact NP oligomers accumulate in cells and degrade. Under weak dissociating conditions, intracellular NP multimers are relatively stable in avian, human A(H2N2) and human A(H3N2) viruses and unstable in human A(H1N1) viruses, dissociating into monomers. NP multimers presumably serve to bring nascent unfolded NP molecules into close contact with each other for further oligomerization, to protect NP monomers from proteolysis, and to serve as intermediates in the posttranslational folding of NP.