Sumoylation is Largely Dispensable for Normal Growth but Facilitates Heat Tolerance in Yeast.

Numerous proteins are sumoylated in normally growing yeast and SUMO conjugation levels rise upon exposure to several stress conditions. We observe high levels of sumoylation also during early exponential growth and when nutrient-rich medium is used. However, we find that reduced sumoylation (∼75% less than normal) is remarkably well-tolerated, with no apparent growth defects under nonstress conditions or under osmotic, oxidative, or ethanol stresses. In contrast, strains with reduced activity of Ubc9, the sole SUMO conjugase, are temperature-sensitive, implicating sumoylation in the heat stress response, specifically. Aligned with this, a mild heat shock triggers increased sumoylation which requires functional levels of Ubc9, but likely also depends on decreased desumoylation, since heat shock reduces protein levels of Ulp1, the major SUMO protease. Furthermore, we find that a ubc9 mutant strain with only ∼5% of normal sumoylation levels shows a modest growth defect, has abnormal genomic distribution of RNA polymerase II (RNAPII), and displays a greatly expanded redistribution of RNAPII after heat shock. Together, our data implies that SUMO conjugations are largely dispensable under normal conditions, but a threshold level of Ubc9 activity is needed to maintain transcriptional control and to modulate the redistribution of RNAPII and promote survival when temperatures rise.

Regulation of transcription factors by sumoylation.

Transcription factors (TFs) are among the most frequently detected targets of sumoylation, and effects of the modification have been studied for about 200 individual TFs to date. TF sumoylation is most often associated with reduced target gene expression, which can be mediated by enhanced interactions with corepressors or by interference with protein modifications that promote transcription. However, recent studies show that sumoylation also regulates gene expression by controlling the levels of TFs that are associated with chromatin. SUMO can mediate this by modulating TF DNA-binding activity, promoting clearance of TFs from chromatin, or indirectly, by influencing TF abundance or localization.

HIV type 1 integrase inhibitors: from basic research to clinical implications.

Similar to other retroviruses, productive infection with HIV-1 requires three key steps in the viral replication: (i) reverse transcription of viral genomic RNA into viral cDNA by the viral reverse transcriptase; (ii) integration of viral cDNA into host cell genome using the viral integrase; and (iii) cleavage of newly synthesized viral polypeptide by the viral protease into individual viral proteins during new virion assembly. Following their discovery, all three viral enzymes were considered as targets for antiretroviral drugs. However, while multiple reverse transcriptase and protease inhibitors have been used for more than 12 years to treat HIV-infected individuals, only recently has the viral integrase enzyme emerged as an alternative, clinically validated target to block HIV-1 replication. Here we review the biology of HIV-1 integration, the mechanisms of action and development of resistance to integrase inhibitors, and the latest data on the most recent clinical trials involving this promising, novel class of antiretroviral drugs.

B cell stimulatory effects of alpha-enolase that is differentially expressed in NZB mouse B cells.

Intrinsic polyclonal B cell activation is characteristic of NZB mice and it contributes to the development of lupus nephritis in NZB crosses. Although multiple autosomal genes appear to be involved, the major loci for B cell hyperactivity have been mapped on chromosome 4. To identify various genes determining B cell hyperactivity, differential mRNA display was done comparing B cells of NZB and BALB/c mice. The approach yielded 32 genes that were consistently upregulated in NZB B cells. Among these, alpha-enolase, which is located in the region of chromosome 4 containing B cell hyperactivity loci, was found to be spontaneously overexpressed only in NZB B cells, but not in splenic B or T cells of BALB/c or T cells of NZB mice. Exposure to soluble, but not plate-bound, enolase induced splenic B cells from normal BALB/c mice or B cell lymphoma lines to secrete Ig that was mediated by augmented transcription. Moreover, in combination with a subthreshold stimulus with anti-IgM, enolase augmented the expression of CD69 and B7.2 in nai;ve B cells from normal mice. Enolase probably functions intracellularly as an accessory molecule in stimulating B cells. Since functionally related genes tend to congregate, enolase may contribute to polyclonal B cell activation in cooperation with other genes in the hyperactivity loci, which appear to be in a transcriptionally active region in NZB B cells.