Biomedical subjects

Fabio Beltram

Publications and source records attributed to Fabio Beltram.

7 recordsLinked to original sources

PubMed2004

Nuclear organization and the control of HIV-1 transcription.

The regulation of transcription of the human immunodeficiency virus (HIV) is a complex event of significant pathological relevance, which recapitulates general concepts of cellular transcription with some peculiarities. The viral promoter is embedded in a chromatin structure that exerts powerful repression on transcription; activation of gene expression relies on the combined activity of a series of cellular factors that respond to different external stimuli, and on the function of a single viral regulatory protein, the Tat transactivator. Transcriptional activation is consequent to both chromatin remodeling and to the recruitment of elongation-competent RNA polymerase II complexes onto the integrated promoter, two events that require the coordinate, but transient, assembly of different protein complexes. Application of optical imaging techniques now allows us to appreciate the spatial and temporal evolvement of these reactions in vivo. The picture that is emerging is not only descriptive, but also relevant to the understanding of the regulation of the process. In particular, it appears that the confinement of biomolecules within specific subcellular compartments represents a way to control and coordinate the assembly of functional complexes that regulate viral gene expression.

Cell Nucleus↗

PubMed2003

Cell membrane lipid rafts mediate caveolar endocytosis of HIV-1 Tat fusion proteins.

The transactivator protein of human immunodeficiency virus type 1 Tat has the unique property of mediating the delivery of large protein cargoes into the cells when present in the extracellular milieu. Here we show that Tat fusion proteins are internalized by the cells through a temperature-dependent endocytic pathway that originates from cell membrane lipid rafts and follows caveolar endocytosis. These conclusions are supported by the study of the slow kinetics of the internalization of Tat endosomes, by their resistance to nonionic detergents, the colocalization of internalized Tat with markers of caveolar endocytosis, and the impairment of the internalization process by drugs that disrupt lipid rafts or disturb caveolar trafficking. These results are of interest for all those who exploit Tat as a vehicle for transcellular protein delivery.

Cell Line↗

PubMed2003

Low-threshold quantum-cascade lasers at 3.5 THz (lambda = 85 microm).

Chirped-superlattice quantum-cascade lasers are reported that emit at lambda approximately 85 microm (3.6 THz), which is to the authors' knowledge the longest wavelength demonstrated so far with this technology. Collected peak output powers of 1.5 mW per facet were measured at liquid-helium temperature, and a maximum operating temperature of 45 K was reached. Record low-threshold-current densities of 95 and 115 A cm(-2) were observed in pulsed and continuous-wave operation, respectively. For the latter, output powers of a few hundred microwatts are estimated at low temperatures.

Journal Article↗

PubMed2003

Recruitment of human cyclin T1 to nuclear bodies through direct interaction with the PML protein.

Human cyclin T1, the cyclin partner of Cdk9 kinase in the positive transcription elongation factor b (P-TEFb), is an essential cellular cofactor that is recruited by the human immunodeficiency virus type 1 (HIV-1) Tat transactivator to promote transcriptional elongation from the HIV-1 long terminal repeat (LTR). Here we exploit fluorescence resonance energy transfer (FRET) to demonstrate that cyclin T1 physically interacts in vivo with the promyelocytic leukaemia (PML) protein within specific subnuclear compartments that are coincident with PML nuclear bodies. Deletion mutants at the C-terminal region of cyclin T1 are negative for FRET with PML and fail to localize to nuclear bodies. Cyclin T1 and PML are also found associated outside of nuclear bodies, and both proteins are present at the chromatinized HIV-1 LTR promoter upon Tat transactivation. Taken together these results suggest that PML proteins regulate Tat- mediated transcriptional activation by modulating the availability of cyclin T1 and other essential cofactors to the transcription machinery.

Cell Nucleus↗

PubMed2003

Nonlinear quasiparticle tunneling between fractional quantum hall edges.

Remarkable nonlinearities in the differential tunneling conductance between fractional quantum Hall edge states at a constriction are observed in the weak-backscattering regime. In the nu=1/3 state a peak develops as temperature is increased and its width is determined by the fractional charge. In the range 2/3</=nu</=1/3 this width displays a symmetric behavior around nu=1/2. We discuss the consistency of these results with available theoretical predictions for interedge quasiparticle tunneling in the weak-backscattering regime.

Journal Article↗

PubMed2003

Caveolae-mediated internalization of extracellular HIV-1 tat fusion proteins visualized in real time.

The Tat protein from HIV-1, when fused with heterologous proteins or peptides, can traverse cell membranes. This ability has generated great interest due to potential therapeutic applications. However, the relevant cellular pathway and its dynamics have not been elucidated yet. Here we unravel the intracellular fate of exogenously added Tat fused with green fluorescent protein (GFP) in live HeLa and CHO cells, from the early interaction with the plasma membrane up to the long-term accumulation in the perinuclear region. We demonstrate that the internalization process of full-length Tat and of heterologous proteins fused to the transduction domain of Tat exploits a caveolar-mediated pathway and is inhibited at 4 degrees C. Remarkably, a slow linear movement toward the nucleus of individual GFP-tagged Tat-filled caveolae with an average velocity of 3 micro m/h was observed. No fluorescence was observed in the nucleus, possibly suggesting that Tat fusion protein unfolding is required for nuclear translocation. In addition, early sensitivity to cytochalasin-D treatment indicates the essential role of the actin cytoskeleton in the displacement of Tat vesicles toward the nucleus. Our results imply that HIV-1 Tat mediates the internalization of protein cargos in a slow and temperature-dependent manner by exploiting the caveolar pathway.

Animals↗

PubMed2002

Terahertz semiconductor-heterostructure laser.

Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.

Journal Article↗