Protein folding is a fundamental process in biology, key to understanding many human diseases. Experimentally, proteins often appear to fold via simple two- or three-state mechanisms involving mainly native-state interactions, yet recent network models built from atomistic simulations of small proteins suggest the existence of many possible metastable states and folding pathways. We reconcile these two pictures in a combined experimental and simulation study of acyl-coenzyme A-binding protein (ACBP), a two-state folder (folding time ~10 ms) exhibiting residual unfolded-state structure, and a putative early folding intermediate. Using single-molecule FRET in conjunction with side-chain mutagenises, we first demonstrate that the denatured state of ACBP at near-zero denaturant is unusually compact and enriched in long-range structure that can be perturbed by discrete hydrophobic core mutations. We then employ ultrafast laminar-flow mixing experiments to study the folding kinetics of ACBP on the microsecond timescale. These studies, along with with Trp-Cys quenching measurements of unfolded-state dynamics, suggest that unfolded-state structure forms on a surprisingly slow (~100 µs) timescale, and that sequence mutations strikingly perturb both time-resolved and equilibrium smFRET measurements in a similar way. A Markov State Model (MSM) of the ACBP folding reaction, constructed from over 30 milliseconds of molecular dynamics trajectory data, predicts a complex network of metastable stables, residual unfolded-state structure and kinetics consistent with experiment, but no well-defined intermediate preceding the main folding barrier. Taken together, these experimental and simulation results suggest that the previously characterized fast kinetic phase is not due to formation of a barrier-limited intermediate, but rather a more heterogeneous and slow acquisition of unfolded-state structure.

We have investigated the multidimensionality of the free energy landscape accessible to a nucleic acid hairpin by measuring the relaxation kinetics in response to two very different perturbations of the folding/unfolding equilibrium, either a laser temperature-jump or ion-jump (from rapid mixing with counterions). The two sets of measurements carried out on DNA hairpins (4 or 5 base pairs in the stem and 21-nucleotide polythymine loop), using FRET between end labels or fluorescence of 2-aminopurine in the stem as conformational probes, yield distinctly different relaxation kinetics in the temperature range 10-30 °C and salt range 100-500 mM NaCl, with rapid mixing exhibiting slower relaxation kinetics after an initial collapse of the chain within 8 μs of the counterion mixing time. The discrepancy in the relaxation times increases with increasing temperatures, with rapid mixing times nearly 10-fold slower than T-jump times at 30 °C. These results rule out a simple two-state scenario with the folded and unfolded ensemble separated by a significant free energy barrier, even at temperatures close to the thermal melting temperature T(m). Instead, our results point to the scenario in which the conformational ensemble accessed after counterion condensation and collapse of the chain is distinctly different from the unfolded ensemble accessed with T-jump perturbation. Our data suggest that, even at temperatures in the vicinity of T(m) or higher, the relaxation kinetics obtained from the ion-jump measurements are dominated by the escape from the collapsed state accessed after counterion condensation.

High-risk HPV is clearly associated with cervical cancer. HPV integration has been confirmed to promote carcinogenesis in the previous studies. In our study, a total of 285 DNA breakpoints and 287 RNA breakpoints were collected. We analyzed the characteristic of HPV integration in the DNA and RNA samples. The results revealed that the patterns of HPV integration in RNA and DNA samples differ significantly. FHIT, KLF5, and LINC00392 were the hotspot genes integrated by HPV in the DNA samples. RAD51B, CASC8, CASC21, ERBB2, TP63, TEX41, RAP2B, and MYC were the hotspot genes integrated by HPV in RNA samples. Breakpoints of DNA samples were significantly prone to the region of INTRON (P < 0.01, Chi-squared test), whereas in the RNA samples, the breakpoints were prone to EXON. Pathway analysis had revealed that the breakpoints of RNA samples were enriched in the pathways of transcriptional misregulation in cancer, cancer pathway, and pathway of adherens junction. Breakpoints of DNA samples were enriched in the pathway of cholinergic synapse. In summary, our data helped to gain insights into the HPV integration sites in DNA and RNA samples of cervical cancer. It had provided theoretical basis for understanding the mechanism of tumorigenesis from the perspective of HPV integration in the HPV-associated cervical cancers.