Neurological disorders develop in most people infected with human immunodeficiency virus type 1 (HIV-1). However, the underlying mechanisms remain largely unknown. Here we report that binding of HIV-1 transactivator (Tat) protein to low-density lipoprotein receptor-related protein (LRP) promoted efficient uptake of Tat into neurons. LRP-mediated uptake of Tat was followed by translocation to the neuronal nucleus. Furthermore, the binding of Tat to LRP resulted in substantial inhibition of neuronal binding, uptake and degradation of physiological ligands for LRP, including alpha2-macroglobulin, apolipoprotein E4, amyloid precursor protein and amyloid beta-protein. In a model of macaques infected with a chimeric strain of simian-human immunodeficiency virus, increased staining of amyloid precursor protein was associated with Tat expression in the brains of simian-human immunodeficiency virus-infected macaques with encephalitis. These results indicate that HIV-1 Tat may mediate HIV-1-induced neuropathology through a pathway involving disruption of the metabolic balance of LRP ligands and direct activation of neuronal genes.

Primary T helper 2 cells are heterogeneous, expressing subsets of cytokines at varying levels. Mechanisms controlling this spectrum of phenotypes are still unclear. The ETS family transcription factor PU.1 is expressed in Th2 but not Th1 cells. Th2 cytokine production is decreased in cultures transduced with a PU.1-expressing retrovirus and increased in Th2 cells following RNAi that decreases PU.1 expression. In primary cultures, PU.1 expression is restricted to a subpopulation of Th2 cells that express CCL22 and a subset of Th2 cytokines. PU.1 regulates the Th2 phenotype by interfering with GATA-3 DNA binding without altering GATA-3 protein levels. Thus, the expression of PU.1 in subsets of Th2 cells establishes a defined cytokine profile and contributes towards establishing the spectrum of cytokine production observed in Th2 populations.

Histone-lysine methylation is linked to transcriptional regulation and the control of epigenetic inheritance. Lysine residues can be mono-, di-, or trimethylated, and it has been suggested that each methylation state of a given lysine may impart a unique biological function. In yeast, histone H3 lysine 4 (K4) is mono-, di-, and trimethylated by the Set1 histone methyltransferase. Previous studies show that Set1 associates with RNA polymerase II and demarcates transcriptionally active genes with K4 trimethylation. To determine whether K4 trimethylation might be selectively regulated, we screened a library of yeast deletion mutants associated with transcriptional regulation and chromatin function. We identified BUR2, a cyclin for the Bur1/2 (BUR) cyclin-dependent protein kinase, as a specific regulator of K4 trimethylation. Surprisingly, BUR also regulated H2B monoubiquitylation, whereas other K4 methylation states and H3 lysine 79 (K79) methylation were unaffected. Synthetic genetic array (SGA) and transcription microarray analyses of a BUR2 mutant revealed that BUR is functionally similar to the PAF, Rad6, and Set1 complexes. These data suggest that BUR acts upstream of these factors to control their function. In support, we show that recruitment of the PAF elongation complex to genes is significantly impaired in a BUR2 deletion. Our data reveal a novel function for the BUR kinase in transcriptional regulation through the selective control of histone modifications.

The proteasome regulates histone lysine methylation and gene transcription, but how it does so is poorly understood. To better understand this process, we used the epistatic miniarray profile (E-MAP) approach to identify factors that genetically interact with proteasomal subunits. In addition to members of the Set1 complex that mediate histone H3 lysine 4 methylation (H3K4me), we found that deleting members of the CCR4/NOT mRNA processing complex exhibit synthetic phenotypes when combined with proteasome mutants. Further biochemical analyses revealed physical associations between CCR4/NOT and the proteasome in vivo. Consistent with the genetic and biochemical interactions linking CCR4/NOT with proteasome and Set1-mediated methylation, we find that loss of Not4 decreases global and gene-specific H3K4 trimethylation (H3K4me3) and decreases 19S proteasome recruitment to the PMA1 gene. Similar to proteasome regulation of histone methylation, loss of CCR4/NOT members does not affect ubiquitinated H2B. Mapping of Not4 identified the RING finger domain as essential for H3K4me3, suggesting a role for ubiquitin in this process. Consistent with this idea, loss of the Not4-interacting protein Ubc4, a known ubiquitin-conjugating enzyme, decreases H3K4me3. These studies implicate CCR4/NOT in the regulation of H3K4me3 through a ubiquitin-dependent pathway that likely involves the proteasome.