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Suramin inhibits the binding of a variety of growth factors to their cell surface receptors. The direct interaction of suramin with acidic fibroblast growth factor has been detected by the enhancement of the drug's fluorescence in the presence of the protein with the maximum effect occurring at a molar ratio of suramin to aFGF of 2:1. This interaction stabilizes aFGF to thermal denaturation and partially protects a free thiol in its polyanion binding site from oxidation. The binding of suramin to aFGF also induces aggregation of the growth factor to at least a hexameric state as detected by static and dynamic light scattering as well as by gel filtration studies. Both CD and amide I' FTIR spectra of aFGF in the presence and absence of suramin suggest that the drug may also be causing a small conformational change in the growth factor. Suramin produces an even greater aggregation of bFGF and PDGF but not of EGF or IGF-1. Evidence for a suramin-induced conformational change in IGF-1 but not EGF is found by CD, however. It is concluded that suramin binds to many growth factors and that this induces microaggregation and, in some cases, conformational changes. In the case of aFGF, suramin interacts at or near its heparin binding site. The relationship between these phenomena and the anti-growth factor activity of suramin remains to be clearly elucidated.

We have examined the equilibrium unfolding of Escherichia coli ribonuclease HI (RNase H), a member of a family of enzymes that cleaves RNA from RNA:DNA hybrids. A completely synthetic gene was constructed that expresses a variant of the wild-type sequence with all 3 cysteines replaced with alanine. The resulting recombinant protein is active and folds reversibly. Denaturation studies monitored by circular dichroism and tryptophan fluorescence yield coincident curves that suggest the equilibrium unfolding reaction is a 2-state process. Acid denaturation, however, reveals a cooperative transition at -pH l .8 to a partially folded state. This acid state can be further denatured in a reversible manner by the addition of heat or urea as monitored by either CD or tryptophan fluorescence.Analytical ultracentrifugation studies indicate that the acid state of RNase H is both compact and monomeric. Although compact, the acid state does not resemble the native protein: the acid state displays a near-UV CD spectrum similar to the unfolded state and binds to and enhances the fluorescence of the dye 1-anilinonaphthalene, 8-sulfonate much more than either the native or unfolded states. Therefore, the acid state of E. coli RNase H has the characteristics of a molten globule: it retains a high degree of secondary structure, remains compact, yet does not appear to contain a tightly packed core.Keywords: acid state; molten globule; protein folding; ribonuclease H The native conformation of a protein arises from many interdependent coupled interactions. A number of forces act in a cooperative manner to increase the stability of the folded state under native conditions. Therefore, in order to understand protein stability fully, we must understand which interactions are most important. This has proven to be difficult because the energetic importance of any particular residue is dependent on its sequence and structural context. Similarly, the cooperativity of the folding process also makes the study of transient intermediates difficult. One approach to circumvent these problems imposed by cooperativity is to study equilibrium conformations of the molecule that are only partially folded. Such intermediate states will yield important information on the range of contexts in which the same amino acid sequence will fold into a stable structure and should reveal which interactions are driving the folding process. These equilibrium intermediates may also serve as good experimental models for transiently populated intermediates in the kinetic pathway of folding.