Natively Unfolded Human Prothymosin α Adopts Partially Folded Collapsed Conformation at Acidic pH

Abstract

Prothymosin alpha has previously been shown to be unfolded at neutral pH, thus belonging to a growing family of "natively unfolded" proteins. The structural properties and conformational stability of recombinant human prothymosin alpha were characterized at neutral and acidic pH by gel filtration, SAXS, circular dichroism, ANS fluorescence, (1)H NMR, and resistance to urea-induced unfolding. Interestingly, prothymosin alpha underwent a cooperative transition from the unfolded state into a partially folded conformation on lowering the pH. This conformation of prothymosin alpha is a compact denatured state, with structural properties different from those of the molten globule. The formation of alpha-helical structure by the glutamic acid-rich elements of the protein accompanied by the partial hydrophobic collapse is expected at lower pH due to the neutralization of the negatively charged residues. It is possible that such conformational changes may be associated with the protein function.

Random-coil behavior and the dimensions of chemically unfolded proteins

“…One is the prosthetic group containing cytochrome c, which, despite containing a very electron-dense iron (leading to an anomalous reduction in R G ), falls quite close to the best-fit line (27) (observed, 30.2 Ϯ 0.2 Å; expected, 30.9 Å). We have also excluded the intrinsically unfolded proteins prothymosin-␣, the carboxyl-terminal domain of caldesmon, ␣-synuclein, ␤-synuclein, and ␥-synuclein (39,24,25). In water, all five are expanded relative to the chemically denatured states of normally well folded proteins (observed͞expected R G : 37.8 Ϯ 0.9͞32.0, 40.8 Ϯ 0.8͞36.1, 40 Ϯ 1͞36.9, 49 Ϯ 1͞35.8, and 61 Ϯ 1͞34.7 Å, respectively).…”

Prothymosin-α inhibits HIV-1 via Toll-like receptor 4-mediated type I interferon induction

“…The phosphate groups of LPS also form ionic interactions with positively charged residues in TLR4 that contribute to multimerization of TLR4 and MD-2 (28). ProTα is highly negatively charged, even at neutral pH values, and exists in a random coil conformation under physiological conditions (29,30). Based on these features, we hypothesize that ProTα has complementary interactions with the cationic clusters of MD-2 and, possibly, TLR4, and its highly flexible nature allows it to adopt a conformation matching that of the LPS binding site.…”

Paradoxes and wonders of intrinsic disorder: Stability of instability

“…78 Prothymosin a is not an exception, and several other extended IDPs, such as p21, p27, a-synuclein, and phosphodiesterase g subunit, were shown to possess high resistance toward heat denaturation and aggregation, being virtually unaltered by heating to 90 C. [78][79][80][81][82][83][84][85][86][87][88] Curiously, this resistance to thermal aggregation has been used for purification of these proteins, 83,[89][90][91][92] and the indifference to heat treatment was proposed as an analytical tool for evaluation of the abundance of extended IDPs in various proteomes. 93,94 Furthermore, extended IDPs, being characterized by high percentages of charged residues and low overall hydrophobicity, do not undergo large-scale structural changes at low pH 95 and remain soluble under these extreme conditions.…”