The c-myc proto-oncogene product, Myc, is a transcription factor that binds thousands of genomic loci 1 . Recent work suggested that rather than up-and down-regulating selected groups of genes 1-3 , Myc targets all active promoters and enhancers in the genome (a phenomenon termed "invasion") and acts as a general amplifier of transcription 4,5 . However, the available data did not readily discriminate between direct and indirect effects of Myc on RNA biogenesis. We addressed this issue with genome-wide chromatin immunoprecipitation and RNA expression profiles during Bcell lymphomagenesis in mice, in cultured B-cells and fibroblasts. Consistent with long-standing observations 6 , we detected general increases in total RNA or mRNA copies per cell (hereby termed "amplification") 4,5 when comparing actively proliferating cells with control quiescent cells: this was true whether cells were stimulated by mitogens (requiring endogenous Myc for a proliferative response) 7,8 or by deregulated, oncogenic Myc activity. RNA amplification and promoter/enhancer invasion by Myc were separable phenomena that could occur without one another. Moreover, whether or not associated with RNA amplification, Myc drove the differential expression of distinct subsets of target genes. Hence, while having the potential to interact with all active/poised regulatory elements in the genome 4,5,9-11 , Myc does not directly act as a global *

Induction of effective antibody responses against HIV-1 infection remains an elusive goal for vaccine development. Progress may require in-depth understanding of the molecular mechanisms of neutralization by monoclonal antibodies. We have analyzed the molecular actions of two rare, broadly neutralizing, human monoclonal antibodies, 4E10 and 2F5, which target the transiently exposed epitopes in the membrane proximal external region (MPER) of HIV-1 gp41 envelope during viral entry. Both have long CDR H3 loops with a hydrophobic surface facing away from the peptide epitope. We find that the hydrophobic residues of 4E10 mediate a reversible attachment to the viral membrane and that they are essential for neutralization, but not for interaction with gp41. We propose that these antibodies associate with the viral membrane in a required first step and are thereby poised to capture the transient gp41 fusion intermediate. These results bear directly on strategies for rational design of HIV-1 envelope immunogens.gp41 ͉ MPER

Most antibodies induced by HIV-1 are ineffective at preventing initiation or spread of infection because they are either nonneutralizing or narrowly isolate-specific. Rare, ''broadly neutralizing'' antibodies have been detected that recognize relatively conserved regions on the envelope glycoprotein. Using stringently characterized, homogeneous preparations of trimeric HIV-1 envelope protein in relevant conformations, we have analyzed the molecular mechanism of neutralization by two of these antibodies, 2F5 and 4E10. We find that their epitopes, in the membrane-proximal segment of the envelope protein ectodomain, are exposed only on a form designed to mimic an intermediate state during viral entry. These results help explain the rarity of 2F5-and 4E10-like antibody responses and suggest a strategy for eliciting them. envelope glycoprotein ͉ membrane fusion H IV-1 infection generally induces a strong antibody response to the envelope glycoprotein [trimeric (gp160) 3 , cleaved to (gp120/gp41) 3 ], the sole antigen on the virion surface. Most induced antibodies are ineffective in preventing infection, however, because they are either nonneutralizing or narrowly isolate-specific, and the virus replicates so rapidly that ongoing selection of neutralization resistant mutants allows viral evolution to ''keep ahead'' of highaffinity antibody production (1). Moreover, much of the antibody response may be to rearranged or dissociated forms of gp120 and gp41, on which the dominant epitopes may be either in hypervariable loops or in positions occluded on virion-borne envelope trimer. Rare, ''broadly neutralizing'' antibodies have been detected that recognize one of three relatively conserved regions on the envelope protein: the CD4-binding site (mAb b12) (2); carbohydrates on the outer gp120 surface (mAb 2G12) (3); and a segment of the gp41 ectodomain adjacent to the viral membrane (mAbs 2F5 and 4E10) (4, 5), often called the ''membrane-proximal external region'' (MPER). We seek to understand the molecular mechanisms of neutralization by these and other antibodies.Fusion of viral and target-cell membranes initiates HIV-1 infection. Conformational changes in gp120 that accompany its binding to receptor (CD4) and coreceptor (e.g., CCR5 or CXCR4) lead to dissociation of gp120 from gp41 and a cascade of refolding events in the latter (6). In the course of these rearrangements, the N-terminal fusion peptide of gp41 translocates and inserts into the target-cell membrane. A proposed extended conformation of the gp41 ectodomain, with its fusion peptide thus inserted and the transmembrane anchor still in the viral membrane, has been called the ''prehairpin intermediate'' (7). It is the target of various fusion inhibitors, including T-20/enfuvirtide, the first approved fusioninhibiting antiviral drug (8), and the characteristics of the intermediate have been deduced from the properties of these inhibitors or mimicries by short gp41 fragments (9). Subsequent rearrangements from the intermediate to the postfusion state of gp41 involve ...