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Currently, delivery of expression vectors, proteins, and/or pharmacologically important peptidyl mimetics to target cells is problematic because of the low percentage of cells targeted, overexpression, size constraints, and bioavailability. Concentration-dependent transduction of full-length proteins and domains directly into cells would serve to alleviate these problems. Previous researchers have demonstrated the ability of proteins linked to the human immunodeficiency virus (HIV) Tat transduction domain to transduce into cells; but because of inefficiencies, this methodological potential has not significantly progressed since 1988. We describe, in this chapter, a significant increase in transduction efficiency of proteins and ease of use by (1) generation of a Tat protein transduction domain in-frame bacterial expression vector, pTAT-HA, and (2) development of a purification protocol yielding denatured proteins. We have transduced full-length Tat fusion proteins ranging in size from 15 to 115 kDa into approximately 100% of all target cells examined, including peripheral blood lymphocytes, all cells present in whole blood, bone marrow stem cells, diploid fibroblasts, fibrosarcoma cells, and keratinocytes. Transduction occurs in a concentration-dependent manner, achieving maximum intracellular concentrations in less than 10 min. We conclude that our methodology generates highly efficient transducible proteins that are biologically active and have broad potential in the manipulation of biological experimental systems, such as apoptotic induction, cell cycle progression, and differentiation, and in the delivery of pharmacologically relevant proteins.

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Transforming growth factor beta (TGF-beta)-mediated G(1) arrest previously has been shown to specifically target inactivation of cyclin D:cyclin-dependent kinase (Cdk) 4/6 complexes. We report here that TGF-beta-treated human HepG2 hepatocellular carcinoma cells arrest in G(1), but retain continued cyclin D:Cdk4/6 activity and active, hypophosphorylated retinoblastoma tumor suppressor protein. Consistent with this observation, TGF-beta-treated cells failed to induce p15(INK4b), down-regulate CDC25A, or increase levels of p21(CIP1), p27(KIP1), and p57(KIP2). However, TGF-beta treatment resulted in the specific inactivation of cyclin E:Cdk2 complexes caused by absence of the activating Thr(160) phosphorylation on Cdk2. Whole-cell lysates from TGF-beta-treated cells showed inhibition of Cdk2 Thr(160) Cdk activating kinase (CAK) activity; however, cyclin H:Cdk7 activity, a previously assumed mammalian CAK, was not altered. Saccharomyces cerevisiae contains a genetically and biochemically proven CAK gene, CAK1, that encodes a monomeric 44-kDa Cak1p protein unrelated to Cdk7. Anti-Cak1p antibodies cross-reacted with a 45-kDa human protein with CAK activity that was specifically down-regulated in response to TGF-beta treatment. Taken together, these observations demonstrate that TGF-beta signaling mediates a G(1) arrest in HepG2 cells by targeting Cdk2 CAK and suggests the presence of at least two mammalian CAKs: one specific for Cdk2 and one for Cdk4/6.

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Delivery of therapeutic proteins into tissues and across the blood-brain barrier is severely limited by the size and biochemical properties of the proteins. Here it is shown that intraperitoneal injection of the 120-kilodalton beta-galactosidase protein, fused to the protein transduction domain from the human immunodeficiency virus TAT protein, results in delivery of the biologically active fusion protein to all tissues in mice, including the brain. These results open new possibilities for direct delivery of proteins into patients in the context of protein therapy, as well as for epigenetic experimentation with model organisms.

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At present, treatment of HIV infection uses small inhibitory molecules that target HIV protease; however, the emergence of resistant HIV strains is increasingly problematic. To circumvent this, we report here a new 'Trojan horse' strategy to kill HIV-infected cells by exploiting HIV protease. We engineered a transducing, modified, apoptosis-promoting caspase-3 protein, TAT-Casp3, that substitutes HIV proteolytic cleavage sites for endogenous ones and efficiently transduces about 100% of cells, but remains inactive in uninfected cells. In HIV-infected cells, TAT-Casp3 becomes processed into an active form by HIV protease, resulting in apoptosis of the infected cell. This strategy could also be applied to other pathogens encoding specific proteases, such as hepatitis C virus, cytomegalovirus and malaria.

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Deletion of antigen-activated T cells after an immune response and during peripheral negative selection after strong T cell receptor (TCR) engagement of cycling T cells occurs by an apoptotic process termed TCR antigen-induced cell death (AID). By analyzing the timing of death, cell cycle markers, BrdU-labeled S phase cells, and phase-specific centrifugally elutriated cultures from stimulated Jurkat T cells and peripheral blood lymphocytes, we found that AID occurs from a late G1 check point prior to activation of cyclin E:Cdk2 complexes. T cells stimulated to undergo AID can be rescued by effecting an early G1 block by direct transduction of p16INK4a tumor suppressor protein or by inactivation of the retinoblastoma tumor suppressor protein (pRb) by transduced HPV E7 protein. These results suggest that AID occurs from a late G1 death check point in a pRb-dependent fashion.

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In cycling cells, the retinoblastoma protein (pRb) is un- and/or hypo-phosphorylated in early G1 and becomes hyper-phosphorylated in late G1. The role of hypo-phosphorylation and identity of the relevant kinase(s) remains unknown. We show here that hypo-phosphorylated pRb associates with E2F in vivo and is therefore active. Increasing the intracellular concentration of the Cdk4/6 specific inhibitor p15(INK4b) by transforming growth factor beta treatment of keratinocytes results in G1 arrest and loss of hypo-phosphorylated pRb with an increase in unphosphorylated pRb. Conversely, p15(INK4b)-independent transforming growth factor beta-mediated G1 arrest of hepatocellular carcinoma cells results in loss of Cdk2 kinase activity with continued Cdk6 kinase activity and pRb remains only hypo-phosphorylated. Introduction of the Cdk4/6 inhibitor p16(INK4a) protein into cells by fusion to a protein transduction domain also prevents pRb hypo-phosphorylation with an increase in unphosphorylated pRb. We conclude that cyclin D:Cdk4/6 complexes hypo-phosphorylate pRb in early G1 allowing continued E2F binding.

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A YAC library enriched for telomere clones was constructed and screened for the human telomere-specific repeat sequence (TTAGGG). Altogether 196 TYAC library clones were studied: 189 new TYAC clones were isolated, 149 STSs were developed for 132 different TY-ACs, and 39 P1 clones were identified using 19 STSs from 16 of the TYACs. A combination of mapping methods including fluorescence in situ hybridization, somatic cell hybrid panels, clamped homogeneous electric fields, meiotic linkage, and BLASTN sequence analysis was utilized to characterize the resource. Forty-five of the TYACs map to 31 specific telomere regions. Twenty-four linkage markers were developed and mapped within 14 proterminal regions (12 telomeres and 2 terminal bands). The polymorphic markers include 12 microsatellites for 10 telomeres (1q, 2p, 6q, 7q, 10p, 10q, 13q, 14q, 18p, 22q) and the terminal bands of 11q and 12p. Twelve RFLP markers were identified and meiotically mapped to the telomeres of 2q, 7q, 8p, and 14q. Chromosome-specific STSs for 27 telomeres were identified from the 196 TYACs. More than 30,000 nucleotides derived from the TYAC vector-insert junction regions or from regions flanking TYAC microsatellites were compared to reported sequences using BLASTN. In addition to identifying homology with previously reported telomere sequences and human repeat elements, gene sequences and a number of ESTs were found to be highly homologous to the TYAC sequences. These genes include human coagulation factor V (F5), Weel protein tyrosine kinase (WEE1), neurotropic protein tyrosine kinase type 2 (NTRE2), glutathione S-transferase (GST1), and beta tubulin (TUBB). The TYAC/P1 resource, derivative STSs, and polymorphisms constitute an enabling resource to further studies of telomere structure and function and a means for physical and genetic map integration and closure.

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