ATP-sensitive potassium (K ATP ) channels conduct potassium ions across cell membranes and thereby couple cellular energy metabolism to membrane electrical activity. Here, we report the heterologous expression and purification of a functionally active K ATP channel complex composed of pore-forming Kir6.2 and regulatory SUR1 subunits, and determination of its structure at 18 Å resolution by single-particle electron microscopy. The purified channel shows ATP-ase activity similar to that of ATPbinding cassette proteins related to SUR1, and supports Rb þ fluxes when reconstituted into liposomes. It has a compact structure, with four SUR1 subunits embracing a central Kir6.2 tetramer in both transmembrane and cytosolic domains. A cleft between adjacent SUR1s provides a route by which ATP may access its binding site on Kir6.2. The nucleotide-binding domains of adjacent SUR1 appear to interact, and form a large docking platform for cytosolic proteins. The structure, in combination with molecular modelling, suggests how SUR1 interacts with Kir6.2.

in cell-attached patches. 4. In excised patches of membrane, direct application of NO or the NO donor, S-nitroso-Nacetyl penicillamine (SNAP), increased the open probability of KNO1 and KNo2, but cGMP or dibutyryl cGMP had no effect. SNAP had no effect on the open probability of BK channels in excised patches. 5. The reducing agent dithiothreitol and the alkylating agent N-ethylmaleimide blocked NOinduced channel openings. 6. In summary, the hyperpolarization response to NO in smooth muscles may be mediated by multiple K' channels. At least two of these classes of channels may be activated by dual pathways involving direct activation by NO and cGMP-mediated mechanisms.

A significant reduction in worm burden over a 12-month period in helminth-infected children increases the risk of allergen skin sensitization but not of clinical allergic disease. The effect on skin sensitization could not be fully explained by any of the immunological parameters tested.

Helper T cells are critical for protective immunity, CD8 + T-cell memory, and CD4 + recall responses, but whether the same or distinct CD4 + T cells are involved in these responses has not been established. Here we describe two CD4 + T cells, LLO118 and LLO56, specific for an immunodominant Listeria monocytogenes epitope, with dramatically different responses to primary and secondary infection. Comparing in vivo responses, LLO118 T cells proliferate more strongly to primary infection, whereas surprisingly, LLO56 has a superior CD4 + recall response to secondary infection. LLO118 T cells provide more robust help for CD8 + T-cell responses to secondary infection than LLO56. We found no detectable differences in antigen sensitivity, but naive LLO118 T cells have much lower levels of CD5 and their T-cell receptor levels are dramatically down-regulated after their strong primary response. Thus, distinct CD4 + helper T cells are specialized to help either in primary or secondary responses to infection.infectious disease | affinity | T-cell receptor transgenic mice | apoptosis C D4 + T cells are essential for CD8 + memory-cell formation, recall responses, and protective immunity (1-3). Despite the importance of CD4 + T cells in immune responses, studies examining T-cell responses to Listeria monocytogenes have focused primarily on CD8 + T cells (4). CD4 + T cells are not required for the primary CD8 + response to L. monocytogenes, but they play a critical role in the maintenance and survival of CD8 + memory cells (1-5). Memory cell formation is a hallmark of the adaptive immune system and successful memory cells must navigate expansion, contraction, and maintenance (6). Much more has been learned about the pathways for formation of CD8 + memory cells than CD4 + memory cells, but it is clear that the pathways for CD4 + memory formation are distinct (6-8). The parameters of optimal CD4 + help for CD8 + and CD4 + memory formation have not been characterized and represent an opportunity to effectively improve vaccines (9).T cells with shortened exposure to antigen do not successfully differentiate to memory cells (10). In contrast, overstimulation in a situation like chronic infection leads to T-cell exhaustion and ineffective memory cell formation or persistence (11). Thus, there appears to be a "goldilocks" level of T-cell stimulation for effective memory cell formation (6). Differences in the duration or abundance of antigen presentation or costimulatory molecules alter activation levels and T-cell receptor (TCR) levels and regulators on the T cell itself affect activation levels (12). TCR affinity and thymic development can also alter T-cell antigen sensitivity (13). For example, the surface expression of one Tcell-negative regulator of T-cell activation, CD5, correlates with the affinity of that developing TCR for self peptide (14-16). The relationship between effective CD4 + responses and the avidity of the TCR:pMHC interaction is not established and a polyclonal CD4 + T-cell population is not amenable to address the...