Vertebrates constantly remodel bone. The resorption of preexisting bone by osteoclasts and the formation of new bone by osteoblasts is strictly coordinated to maintain bone mass within defined limits. A few molecular determinants of bone remodelling that affect osteoclast activity have been characterized, but the molecular determinants of osteoblast activity are unknown. To investigate the role of osteocalcin, the most abundant osteoblast-specific non-collagenous protein, we have generated osteocalcin-deficient mice. These mice develop a phenotype marked by higher bone mass and bones of improved functional quality. Histomorphometric studies done before and after ovariectomy showed that the absence of osteocalcin leads to an increase in bone formation without impairing bone resorption. To our knowledge, this study provides the first evidence that osteocalcin is a determinant of bone formation.

Infection by HTLV-1 has been correlated with the appearance of various proliferative or degenerative diseases. Some of these disorders have been observed in transgenic mice expressing the Tax protein, which is known to transactivate various viral and cellular promoters through interactions with several transcription factors. In this study we show that the C-terminus of this viral oncoprotein represents a motif permitting binding of Tax to the PDZ domains of several cellular proteins. A two-hybrid screen with Tax as bait indeed yielded complementary DNAs coding for six proteins including PDZ domains. Two of them correspond to truncated forms of the PSD-95 and b1-syntrophin proteins, another clone codes for a protein homologous to the product of the C. elegans gene lin-7. The other three clones code for new human members of the PDZ family of cellular proteins. The interaction of Tax with the products of these clones was con®rmed by immunoprecipitation assays in mammalian cells, and analysis of various mutants of Tax established the importance of the Cterminal amino acids for several of these interactions. These data suggest that Tax could perturb the normal function of targeted cellular proteins by strongly interacting with their PDZ domains.

The Tax transactivator of the human T cell leukemia virus type I (HTLV-I) exhibits oncogenic properties. A screen for proteins interacting with Tax yielded a complementary DNA (cDNA) encoding the human Int-6 protein. In mice, the Int-6 gene can be converted into a putative dominant negative oncogene after retroviral insertion. Here, Int-6 was localized in the cell nucleus to give a speckled staining pattern superposed to that of the promyelocytic leukemia (PML) protein. The binding of Tax to Int-6 caused its redistribution from the nuclear domains to the cytoplasm. Thus, Int-6 is a component of the PML nuclear bodies and Tax disrupts its normal cellular localization by binding to it.

The viral Tax protein, which is encoded by human T-cell leukaemia virus HTLV-I, activates nuclear translocation of the NF-kappa B/Rel transcription factors and relieves cytoplasmic sequestration of RelA and Rel by heterodimerization with NF-kappa B1/p1O5 (refs 1,2). Proteolytic maturation of this precursor protein is performed by the proteasome complex. Here we show that Tax binds specifically to two subunits of the 20S proteasome, HsN3 and HC9. This interaction is weakened with HsN3 and lost for HC9 when a mutant of Tax is substituted that is selectively defective for NF-kappa B activation. Immunoprecipitation shows that p1O5 binds weakly to HC9 and that this interaction is reinforced by Tax. No bridging function of Tax between p1O5 and HsN3 was observed. From these results, we propose that Tax accelerates the proteolytic maturation of P105 by favouring its anchorage to the proteasome.