Processing of the GagPol polyprotein precursor of human immunodeficiency virus type 1 (HIV-1) is a critical step in viral assembly and replication. The HIV-1 protease (PR) is translated as part of GagPol and is both necessary and sufficient for precursor processing. The PR is active only as a dimer; enzyme activation is initiated when the PR domains in two GagPol precursors dimerize. The precise mechanism by which the PR becomes activated and the subsequent initial steps in precursor processing are not well understood. However, it is clear that processing is initiated by the PR domain that is embedded within the precursor itself. We have examined the earliest events in precursor processing using an in vitro assay in which full-length GagPol is cleaved by its embedded PR. We demonstrate that the embedded, immature PR is as much as 10,000-fold less sensitive to inhibition by an active-site PR inhibitor than is the mature, free enzyme. Further, we find that different concentrations of the active-site inhibitor are required to inhibit the processing of different cleavage sites within GagPol. Finally, our results indicate that the first cleavages carried out by the activated PR within GagPol are intramolecular. Overall, our data support a model of virus assembly in which the first cleavages occur in GagPol upstream of the PR. These intramolecular cleavages produce an extended form of PR that completes the final processing steps accompanying the final stages of particle assembly by an intermolecular mechanism.

The TREX1 enzyme processes DNA ends as the major 3 3 5 exonuclease activity in human cells. Mutations in the TREX1 gene are an underlying cause of the neurological brain disease Aicardi-Goutières syndrome implicating TREX1 dysfunction in an aberrant immune response. TREX1 action during apoptosis likely prevents autoimmune reaction to DNA that would otherwise persist. To understand the impact of TREX1 mutations identified in patients with Aicardi-Goutières syndrome on structure and activity we determined the x-ray crystal structure of the dimeric mouse TREX1 protein in substrate and product complexes containing single-stranded DNA and deoxyadenosine monophosphate, respectively. The structures show the specific interactions between the bound nucleotides and the residues lining the binding pocket of the 3 terminal nucleotide within the enzyme active site that account for specificity, and provide the molecular basis for understanding mutations that lead to disease. Three mutant forms of TREX1 protein identified in patients with AicardiGoutières syndrome were prepared and the measured activities show that these specific mutations reduce enzyme activity by 4 -35,000-fold. The structure also reveals an 8-amino acid polyproline II helix within the TREX1 enzyme that suggests a mechanism for interactions of this exonuclease with other protein complexes.Processing of DNA ends is an important step in many DNA metabolic pathways such as replication, repair, and recombination. The 3Ј 3 5Ј exonucleases play a critical role in correcting fragmented, modified, mispaired, or even normal nucleotides to generate 3Ј termini suitable for downstream events. The drastic consequences that result from impaired 3Ј exonuclease activities underscore the importance of these enzymes for cell survival. Proofreading of DNA synthesis by 3Ј exonucleases is one of the major determinants of mutagenesis and genome stability and cells lacking this ability show a high incidence of cancers (1-3) (for review, see Ref. 4). Cells with defects in proteins containing 3Ј exonuclease activity, such as the Werner syndrome protein, MRE11, APE1, and p53 proteins display chromosomal instability, cell cycle checkpoint defects, and sensitivity to ionizing radiation (5-9).The major 3Ј 3 5Ј exonuclease activity detected in human cell extracts is catalyzed by the TREX1 enzyme. The genes encoding the TREX1 and closely related TREX2 proteins have been identified and cloned (10, 11), and the recombinant proteins confirm the robust catalytic nature of these enzymes (12, 13). Amino acid sequence analysis reveals the TREX proteins belong to the DnaQ family of 3Ј 3 5Ј exonucleases; a structurally conserved group of exonucleases that span Archaea and bacteria to humans and includes such proteins as the exonuclease domains of Werner syndrome protein, the bacterial ⑀ subunit of DNA polymerase III (⑀ subunit), and exonuclease I (Exo I) 2 (14 -17). A hallmark of the DnaQ family exonucleases is three conserved sequence motifs known as Exo I, II, and III. These motifs contain four ...

No abstract

. Tubular cell-enriched subpopulation of primary renal cells improves survival and augments kidney function in rodent model of chronic kidney disease. Am J Physiol Renal Physiol 299: F1026 -F1039, 2010. First published September 8, 2010 doi:10.1152/ajprenal.00221.2010.-Established chronic kidney disease (CKD) may be identified by severely impaired renal filtration that ultimately leads to the need for dialysis or kidney transplant. Dialysis addresses only some of the sequelae of CKD, and a significant gap persists between patients needing transplant and available organs, providing impetus for development of new CKD treatment modalities. Some postulate that CKD develops from a progressive imbalance between tissue damage and the kidney's intrinsic repair and regeneration processes. In this study we evaluated the effect of kidney cells, delivered orthotopically by intraparenchymal injection to rodents 4 -7 wk after CKD was established by two-step 5/6 renal mass reduction (NX), on the regeneration of kidney function and architecture as assessed by physiological, tissue, and molecular markers. A proof of concept for the model, cell delivery, and systemic effect was demonstrated with a heterogeneous population of renal cells (UNFX) that contained cells from all major compartments of the kidney. Tubular cells are known contributors to kidney regeneration in situ following acute injury. Initially tested as a control, a tubular cellenriched subpopulation of UNFX (B2) surprisingly outperformed UNFX. Two independent studies (3 and 6 mo in duration) with B2 confirmed that B2 significantly extended survival and improved renal filtration (serum creatinine and blood urea nitrogen). The specificity of B2 effects was verified by direct comparison to cell-free vehicle controls and an equivalent dose of non-B2 cells. Quantitative histological evaluation of kidneys at 6 mo after treatment confirmed that B2 treatment reduced severity of kidney tissue pathology. Treatmentassociated reduction of transforming growth factor (TGF)-␤1, plasminogen activator inhibitor (PAI)-1, and fibronectin (FN) provided evidence that B2 cells attenuated canonical pathways of profibrotic extracellular matrix production. regeneration; stabilization CHRONIC KIDNEY DISEASE (CKD) affects more than 19 million people in the U.S. and frequently develops as a consequence of chronic obesity, diabetes, and/or hypertension (42). Patients in stage 4 -5 CKD receive dialysis and a complex drug regimen, and the number of kidneys available for transplant is vastly insufficient to meet the need (30). New treatments that delay or reduce dialysis dependence are needed to fill this void.Kidney tissue is composed of Ͼ20 specialized cell types structurally organized into morphologically and functionally distinct compartments that act in concert to filter blood, produce urine, and regulate endocrine function and acid-base and electrolyte balance. Cell-cell interactions are critical to kidney function and are at least partially dependent on spatial and architectural relations...

No abstract