Mutation of conserved negatively charged residues in the S2 and S3 transmembrane segments of a mammalian K+ channel selectively modulates channel gating.

Voltage-gated channel proteins sense a change in the transmembrane electric field and respond with a conformational change that allows ions to diffuse across the pore-forming structure. Site-specific mutagenesis combined with electrophysiological analysis of expressed mutants in amphibian oocytes has previously established the S4 transmembrane segment as an element of the voltage sensor. Here, we show that mutations of conserved negatively charged residues in S2 and S3 of a brain K+ channel, thought of as countercharges for the positively charged residues in S4, selectively modulate channel gating without modifying the permeation properties. Mutations of Glu235 in S2 that neutralize or reverse charge increase the probability of channel opening and the apparent gating valence. In contrast, replacements of Glu272 by Arg or Thr268 by Asp in S3 decrease the open probability and the apparent gating valence. Residue Glu225 in S2 tolerated replacement only by acidic residues, whereas Asp258 in S3 was intolerant to any attempted change. These results imply that S2 and S3 are unlikely to be involved in channel lining, yet, together with S4, may be additional components of the voltage-sensing structure.

Detection of one slowly exchanging substrate water molecule in the S3 state of photosystem II.

The exchangeability of the substrate water molecules at the catalytic site of water oxidation in photosystem II has been probed by isotope-exchange measurements using mass spectrometric detection of flash-induced oxygen evolution. A stirred sample chamber was constructed to reduce the lag time between injection of H2(18)O and the detecting flash by a factor of more than 1000 compared to the original experiments by R. Radmer and O. Ollinger [(1986) FEBS Lett. 195, 285-289]. Our data show that there is a slow (t1/2 approximately 500 ms, 10 degrees C) and a fast (t1/2 <25 ms, 10 degrees C) exchanging substrate water molecule in the S3 state of photosystem II. The slow exchange is coupled with an activation energy of about 75 kJ/mol and is discussed in terms of a terminal manganese oxo ligand, while the faster exchanging substrate molecule may represent a water molecule not directly bound to the manganese center.

The report of the committee about Harvard Coledge: presented to the Generall Court held the 3 of (3) month, 1654

Two folio-sized leaves with a handwritten draft of the May 3, 1654 report of a General Court Committee authorized to investigate the financial state of Harvard College. The report responds directly to eight questions raised in the September 10, 1653 Order of the General Court that established the Committee. The report provides summaries of Harvard's income sources and disbursements, offers recommendations regarding the President's salary and the allowances for the academic Fellows, steward, butler, and cook, and indicates specific contributions from local towns.

Bliss, Nathaniel, 1622-1654, autographed letter signed to John Winthrop; Springfield, Mass., 1 side (2 pages), 1652 December 21

Correspondence regarding an illness Bliss was suffering; he writes that the medicines Winthrop had given him were ineffective and he has been suffering fits. The letter, which was finished in an unknown hand, reports further symptoms had developed, including headache and blindness, and requests Winthrop again send instructions for taking the medicine he originally sent Bliss, and any other medicine he would recommend.