DNA bending by hexamethylene-tethered ammonium ions.

DNA is bent when complexed with certain proteins. We are exploring the hypothesis that asymmetric neutralization of phosphate charges will cause the DNA double helix to collapse toward the neutralized face. We have previously shown that DNA spontaneously bends toward one face of the double helix when it is partially substituted with neutral methylphosphonate linkages. We have now synthesized DNA duplexes in which cations are tethered by hexamethylene chains near specific phosphates. Electrophoretic phasing experiments demonstrate that tethering six ammonium ions on one helical face causes DNA to bend by approximately 5 degrees toward that face, in qualitative agreement with predictions. Ion pairing between tethered cations and DNA phosphates provides a new model for simulating the electrostatic consequences of phosphate neutralization by proteins.

A role for intracellular pH in membrane IgM-mediated cell death of human B lymphomas

We show that anti-IgM-induced cell death in a human B lymphoma cell line, B104, is associated with early intracellular acidification and cell shrinkage. In contrast, another human B cell lymphoma line, Daudi, less susceptible to B cell antigen receptor-mediated cell death, responded to anti-IgM with an early increase in intracellular pH (pHi). The anti-IgM-induced changes of pHi were associated with different levels of activation of the Na+/H+ exchanger isoform 1 (NHE1) as judged by its phosphorylation status. Prevention of anti-IgM-induced cell death in B104 cells by the calcineurin phosphatase inhibitor, cyclosporin A, abrogated both intracellular acidification and cell shrinkage and was associated with an increase in the phosphorylation level of NHE1 within the first 60 min of stimulation. This indicates a key role for calcineurin in regulating pHi and cell viability. The potential role of pHi in cell viability was confirmed in Daudi cells treated with an Na+/H+ exchanger inhibitor 5-(N,N-hexamethylene)amiloride. These observations indicate that the outcome of the anti-IgM treatment depends on NHE1-controlled pHi. We suggest that inactivation of the NHE1 in anti-IgM-stimulated cells results in intracellular acidification and subsequently triggers or amplifies cell death.