Quantitative comparison of a human cancer cell surface proteome between interphase and mitosis.

The cell surface is the cellular compartment responsible for communication with the environment. The interior of mammalian cells undergoes dramatic reorganization when cells enter mitosis. These changes are triggered by activation of the CDK1 kinase and have been studied extensively. In contrast, very little is known of the cell surface changes during cell division. We undertook a quantitative proteomic comparison of cell surface-exposed proteins in human cancer cells that were tightly synchronized in mitosis or interphase. Six hundred and twenty-eight surface and surface-associated proteins in HeLa cells were identified; of these, 27 were significantly enriched at the cell surface in mitosis and 37 in interphase. Using imaging techniques, we confirmed the mitosis-selective cell surface localization of protocadherin PCDH7, a member of a family with anti-adhesive roles in embryos. We show that PCDH7 is required for development of full mitotic rounding pressure at the onset of mitosis. Our analysis provided basic information on how cell cycle progression affects the cell surface. It also provides potential pharmacodynamic biomarkers for anti-mitotic cancer chemotherapy.

G-quadruplex formation in telomeres enhances POT1/TPP1 protection against RPA binding.

Human telomeres terminate with a single-stranded 3' G overhang, which can be recognized as a DNA damage site by replication protein A (RPA). The protection of telomeres (POT1)/POT1-interacting protein 1 (TPP1) heterodimer binds specifically to single-stranded telomeric DNA (ssTEL) and protects G overhangs against RPA binding. The G overhang spontaneously folds into various G-quadruplex (GQ) conformations. It remains unclear whether GQ formation affects the ability of POT1/TPP1 to compete against RPA to access ssTEL. Using single-molecule Förster resonance energy transfer, we showed that POT1 stably loads to a minimal DNA sequence adjacent to a folded GQ. At 150 mM K(+), POT1 loading unfolds the antiparallel GQ, as the parallel conformation remains folded. POT1/TPP1 loading blocks RPA's access to both folded and unfolded telomeres by two orders of magnitude. This protection is not observed at 150 mM Na(+), in which ssTEL forms only a less-stable antiparallel GQ. These results suggest that GQ formation of telomeric overhangs may contribute to suppression of DNA damage signals.

RPA-mediated unfolding of systematically varying G-quadruplex structures.

G-quadruplex (GQ) is a noncanonical nucleic acid structure that is formed by guanine rich sequences. Unless it is destabilized by proteins such as replication protein A (RPA), GQ could interfere with DNA metabolic functions, such as replication or repair. We studied RPA-mediated GQ unfolding using single-molecule FRET on two groups of GQ structures that have different loop lengths and different numbers of G-tetrad layers. We observed a linear increase in the steady-state stability of the GQ against RPA-mediated unfolding with increasing number of layers or decreasing loop length. The stability demonstrated by different GQ structures varied by at least three orders of magnitude. Those with shorter loops (less than three nucleotides long) or a greater number of layers (more than three layers) maintained a significant folded population even at physiological RPA concentration (≈1 µM), raising the possibility of physiological viability of such GQ structures. Finally, we measured the transition time between the start and end of the RPA-mediated GQ unfolding process to be 0.35 ± 0.10 s for all GQ constructs we studied, despite significant differences in their steady-state stabilities. We propose a two-step RPA-mediated GQ unfolding mechanism that is consistent with our observations.

Type A Aortic Dissection Presenting as Acute Coronary Syndrome in a Young Male Patient: A Case Report.

Type A aortic dissection (AD) is a devastating cardiovascular emergency requiring emergent surgical intervention. Most patients with AD have several risk factors for the disease including longstanding hypertension, smoking history, atherosclerosis, and old age. Younger patients may also present with AD if a genetic disorder affecting the integrity of the aorta is present. This case presents an otherwise healthy 36-year-old male with no known significant family history who presented with an atypical presentation of aortic dissection. He described a five-day history of chest pressure made worse with exertion followed by progressive dyspnea which prompted him to seek medical attention. His initial laboratory workup revealed an elevated troponin I level which prompted a cardiology consultation in the emergency department. Transthoracic echocardiography revealed dilatation of the aortic root and aortic regurgitation. CT angiography of the chest was performed revealing a type A dissection beginning at the aortic root and terminating proximal to the right brachiocephalic artery. Involvement of the coronary arteries was suspected due to the elevated troponin I. He was taken to the operating room and underwent aortic grafting, right coronary artery bypass, and repair of the left main artery. Unfortunately, at the end of the operation, the patient went into refractory ventricular fibrillation, which progressed to asystole. He was unable to be revived.

Replication protein A unfolds G-quadruplex structures with varying degrees of efficiency.

Replication protein A (RPA) is known to interact with guanine- (G-) rich sequences that adopt G-quadruplex (GQ) structures. Most studies reported in the literature were performed on GQ formed by homogeneous sequences, such as the human telomeric repeat, and RPA's ability to unfold GQ structures of differing stability is not known. We compared the thermal stability of three potential GQ-forming DNA sequences (PQSs) to their stability against RPA-mediated unfolding using single-molecule fluorescence resonance energy transfer (FRET) and bulk biophysical and biochemical experiments. One of these sequences is the human telomeric repeat and the other two, located in the promoter region of tyrosine hydroxylase gene, are highly heterogeneous sequences that better represent PQSs in the genome. The three GQ constructs have thermal stabilities that differ significantly. Our measurements showed that the most thermally stable structure (Tm = 86 °C) was also the most stable against RPA-mediated unfolding, although the least thermally stable structure (Tm = 69 °C) had at least an order-of-magnitude higher stability against RPA-mediated unfolding than the structure with intermediate thermal stability (Tm = 78 °C). The significance of this observation becomes more evident when considered within the context of the cellular environment where protein-DNA interactions can be an important determinant of GQ viability. Considering these results, we conclude that thermal stability is not necessarily an adequate criterion for predicting the physiological viability of GQ structures. Finally, we measured the time it takes for an RPA molecule to unfold a GQ from a fully folded to a fully unfolded conformation using a single-molecule stopped-flow method. All three GQ structures were unfolded within Δt ≈ 0.30 ± 0.10 s, a surprising result considering that the unfolding time does not correlate with thermal stability or stability against RPA-mediated unfolding. These results suggest that the limiting step in G-quadruplex unfolding by RPA is simply the accessibility of the structure to the RPA protein.