Diamond nanoparticles (nanodiamonds) have been recently proposed as new labels for cellular imaging. For small nanodiamonds (size <40 nm), resonant laser scattering and Raman scattering cross sections are too small to allow single nanoparticle observation. Nanodiamonds can, however, be rendered photoluminescent with a perfect photostability at room temperature. Such a remarkable property allows easier single-particle tracking over long time scales. In this work, we use photoluminescent nanodiamonds of size <50 nm for intracellular labeling and investigate the mechanism of their uptake by living cells. By blocking selectively different uptake processes, we show that nanodiamonds enter cells mainly by endocytosis, and converging data indicate that it is clathrin-mediated. We also examine nanodiamond intracellular localization in endocytic vesicles using immunofluorescence and transmission electron microscopy. We find a high degree of colocalization between vesicles and the biggest nanoparticles or aggregates, while the smallest particles appear free in the cytosol. Our results pave the way for the use of photoluminescent nanodiamonds in targeted intracellular labeling or biomolecule delivery.

The 3-processing of the extremities of viral DNA is the first of two reactions catalyzed by HIV-1 integrase (IN). High order IN multimers (tetramers) are required for complete integration, but it remains unclear which oligomer is responsible for the 3-processing reaction. Moreover, IN tends to aggregate, and it is unknown whether the polymerization or aggregation of this enzyme on DNA is detrimental or beneficial for activity. We have developed a fluorescence assay based on anisotropy for monitoring release of the terminal dinucleotide product in realtime. Because the initial anisotropy value obtained after DNA binding and before catalysis depends on the fractional saturation of DNA sites and the size of IN⅐DNA complexes, this approach can be used to study the relationship between activity and binding/multimerization parameters in the same assay. By increasing the IN:DNA ratio, we found that the anisotropy increased but the 3-processing activity displayed a characteristic bell-shaped behavior. The anisotropy values obtained in the first phase were predictive of subsequent activity and accounted for the number of complexes. Interestingly, activity peaked and then decreased in the second phase, whereas anisotropy continued to increase. Time-resolved fluorescence anisotropy studies showed that the most competent form for catalysis corresponds to a dimer bound to one viral DNA end, whereas higher order complexes such as aggregates predominate during the second phase when activity drops off. We conclude that a single IN dimer at each extremity of viral DNA molecules is required for 3-processing, with a dimer of dimers responsible for the subsequent full integration.The integration of a DNA copy of the HIV-1 2 genome into the host genome is a crucial step in the life cycle of the retrovirus. Integrase (IN) is responsible for the two consecutive reactions that constitute the overall integration process. The first of these two reactions is 3Ј-processing, which involves cleavage of the 3Ј-terminal GT dinucleotide at each extremity of the viral DNA. The hydroxyl groups of newly recessed 3Ј-ends are then used in the second reaction, strand transfer, for the covalent joining of viral and target DNAs, resulting in full-site integration. IN is sufficient for catalysis of the 3Ј-processing reaction in vitro, using short-length oligodeoxynucleotides (ODNs) that mimic one viral long terminal repeat (LTR) in the presence of the metallic cofactor Mg 2ϩ . This reaction generates two products: the viral DNA containing the recessed extremity and the GT dinucleotide. One of the two products, the processed viral DNA, as well as the target DNA serve as substrates for the subsequent joining reaction.IN belongs to the superfamily of polynucleotidyl transferases. Its catalytic core domain contains a triad of acidic residues constituting the D,D-35-E motif, which is strictly required for catalysis. The catalytic core establishes specific contacts with the viral DNA and, together with the C-terminal domain, is involved in DNA binding (1-4). ...

Self-assembly properties of HIV-1 integrase were investigated by time-resolved fluorescence anisotropy using tryptophanyl residues as a probe. From simulation analyses, we show that suitable photon counting leads to an accurate determination of long rotational correlation times in the range of 20-80 ns, permitting the distinction of the monomer, dimer, and tetramer from higher oligomeric forms of integrase. The accuracy of correlation times higher than 100 ns is too low to distinguish the octamer from other larger species. The oligomeric states of the widely used detergent-solubilized integrase were then studied in solution under varying parameters known to influence the activity. In the micromolar range, integrase exists as high-order multimers such as an octamer and/or aggregates and a well-defined tetramer, at 25 and 35 degrees C, respectively. However, integrase is monomeric at catalytically active concentrations (in the sub-micromolar range). Detergents (NP-40 and CHAPS) and divalent cation cofactors (Mg(2+) and Mn(2+)) have a clear dissociative effect on the high multimeric forms of integrase. In addition, we observed that Mg(2+) and Mn(2+) have different effects on both the oligomeric state and the conformation of the monomer. This could explain in part why these two metal cations are not equivalent in terms of catalytic activity in vitro. In contrast, addition of Zn(2+) stimulates dimerization. Interestingly, this role of Zn(2+) in the multimerization process was evident only in the presence of Mg(2+) which by itself does not induce oligomerization. Finally, it is highly suggested that the presence of detergent during the purification procedure plays a negative role in the proper self-assembly of integrase. Accordingly, the accompanying paper [Leh, H., et al. (2000) Biochemistry 39, 9285-9294] shows that a detergent-free integrase preparation has self-assembly and catalytic properties different from those of the detergent-solubilized enzyme.