Ezetimibe is a potent inhibitor of cholesterol absorption that has been approved for the treatment of hypercholesterolemia, but its molecular target has been elusive. Using a genetic approach, we recently identified Niemann-Pick C1-Like 1 (NPC1L1) as a critical mediator of cholesterol absorption and an essential component of the ezetimibe-sensitive pathway. To determine whether NPC1L1 is the direct molecular target of ezetimibe, we have developed a binding assay and shown that labeled ezetimibe glucuronide binds specifically to a single site in brush border membranes and to human embryonic kidney 293 cells expressing NPC1L1. Moreover, the binding affinities of ezetimibe and several key analogs to recombinant NPC1L1 are virtually identical to those observed for native enterocyte membranes. KD values of ezetimibe glucuronide for mouse, rat, rhesus monkey, and human NPC1L1 are 12,000, 540, 40, and 220 nM, respectively. Last, ezetimibe no longer binds to membranes from NPC1L1 knockout mice. These results unequivocally establish NPC1L1 as the direct target of ezetimibe and should facilitate efforts to identify the molecular mechanism of cholesterol transport.cholesterol ͉ intestinal brush border membranes

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The faithful recognition of the interstrand hydrogen bonds between complementary nucleobases forms the foundation of the genetic code. The ability to replicate DNA containing a stable third base pair would allow for an expansion of the information content of DNA by supplementing the existing two base pairs of the genetic alphabet with a third. We report the optimization of unnatural nucleobases whose pairing in duplex DNA is based on interbase hydrophobic interactions. We show that the stability and selectivity of such unnatural base pairs may be comparable to, or even exceed, that of native pairs. We also demonstrate that several unnatural base pairs are incorporated into DNA by Klenow fragment of Escherichia coli DNA polymerase I with an efficiency equivalent to that of native DNA synthesis. Moreover, the unnatural bases are orthogonal to the native bases, with correct pairing being favored by at least an order of magnitude relative to mispairing.

The development of new antimicrobial drugs is a priority to combat the increasing spread of multidrug-resistant bacteria. This development is especially problematic in gram-negative bacteria due to the outer membrane (OM) permeability barrier and multidrug efflux pumps. Therefore, we screened for compounds that target essential, nonredundant, surface-exposed processes in gram-negative bacteria. We identified a compound, MRL-494, that inhibits assembly of OM proteins (OMPs) by the β-barrel assembly machine (BAM complex). The BAM complex contains one essential surface-exposed protein, BamA. We constructed a bamA mutagenesis library, screened for resistance to MRL-494, and identified the mutation bamAE470K. BamAE470K restores OMP biogenesis in the presence of MRL-494. The mutant protein has both altered conformation and activity, suggesting it could either inhibit MRL-494 binding or allow BamA to function in the presence of MRL-494. By cellular thermal shift assay (CETSA), we determined that MRL-494 stabilizes BamA and BamAE470K from thermally induced aggregation, indicating direct or proximal binding to both BamA and BamAE470K. Thus, it is the altered activity of BamAE470K responsible for resistance to MRL-494. Strikingly, MRL-494 possesses a second mechanism of action that kills gram-positive organisms. In microbes lacking an OM, MRL-494 lethally disrupts the cytoplasmic membrane. We suggest that the compound cannot disrupt the cytoplasmic membrane of gram-negative bacteria because it cannot penetrate the OM. Instead, MRL-494 inhibits OMP biogenesis from outside the OM by targeting BamA. The identification of a small molecule that inhibits OMP biogenesis at the cell surface represents a distinct class of antibacterial agents.

Six novel unnatural nucleobases have been characterized that form stable base pairs in duplex DNA, relying not on hydrogen bonds, but rather on interbase hydrophobic interactions. These nucleobases are derivatives of the hydrophobic base pair between 7-azaindole (7AI) and isocarbostyril (ICS). Derivatives of 7AI and ICS were examined that have increased hydrophobic surface area, as well as increased polarizability. As observed with 7AI and ICS, these derivatives are recognized as substrates by Klenow fragment of Escherichia coli DNA polymerase I. The unnatural base pair between pyrrolopyrizine (PP) and C3-methylisocarbostyril (MICS) is enzymatically incorporated into DNA with high efficiency (k cat/K M = 106 M-1 min-1) and moderate selectivity. These studies represent a significant step toward the generation of a stable, orthogonal base pair that can be enzymatically incorporated into DNA with good fidelity.