53BP1 protein is re-localized to the sites of DNA damage after ionizing radiation (IR) and is involved in DNA-damage-checkpoint signal transduction. We examined the dynamics of GFP-53BP1 in living cells. The protein starts to accumulate at the sites of DNA damage 2-3 minutes after damage induction. Fluorescence recovery after photobleaching experiments showed that GFP-53BP1 is highly mobile in non-irradiated cells. Upon binding to the IR-induced nuclear foci, the mobility of 53BP1 reduces greatly. The minimum (M) domain of 53BP1 essential for targeting to IR induced foci consists of residues 1220-1703. GFP-M protein forms foci in mouse embryonic fibroblast cells lacking functional endogenous 53BP1. The M domain contains a tandem repeat of Tudor motifs and an arginine- and glycine-rich domain (RG stretch), which are often found in proteins involved in RNA metabolism, the former being essential for targeting. RNase A treatment dissociates 53BP1 from IR-induced foci. In HeLa cells, dissociation of the M domain without the RG stretch by RNase A treatment can be restored by re-addition of nuclear RNA in the early stages of post-irradiation. 53BP1 immunoprecipitates contain some RNA molecules. Our results suggest a possible involvement of RNA in the binding of 53BP1 to chromatin damaged by IR.

Lymphocyte emigration from the blood into most secondary lymphoid organs and chronically inflamed tissues occurs at the level of high endothelial venules (HEV). A unique characteristic of HEV endothelial cells (HEVEC) is their capacity to incorporate large amounts of sulfate into sialomucin-type counter-receptors for the lymphocyte homing receptor L-selectin. We have previously shown that sulfate uptake into HEVEC is mediated by two distinct functional classes of sulfate transporters: Na+-coupled transporters and sulfate/anion exchangers. Here, we report the molecular characterization from human HEVEC of SLC26A11, a novel member of the SLC26 sulfate/anion exchanger family. Functional expression studies in COS-7 and Sf9 insect cells revealed that SLC26A11 is targeted to the cell membrane and exhibits Na+-independent sulfate transport activity, sensitive to the anion exchanger inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Northern blot analysis showed the highest SLC26A11 transcript levels in placenta, kidney, and brain. The SLC26A11 gene mapped to human chromosome 17q25, very close to the hereditary hearing loss diseases loci DFNA20, DFNA26, and USH1G. RT-PCR analysis of SLC26 sulfate transporters in human HEVEC revealed coexpression of SLC26A11 with SLC26A2/DTDST and lack of SLC26A1/SAT1, SLC26A3/DRA, and SLC26A8/TAT1. Together, our results indicate that SLC26A11 is a novel Na+-independent sulfate transporter that may cooperate with SLC26A2 to mediate DIDS-sensitive sulfate uptake into HEVEC.

We have analyzed the expression pattern of the D1 gene and the localization of its product, the AT hookbearing nonhistone chromosomal protein D1, during Drosophila melanogaster development. D1 mRNAs and protein are maternally contributed, and the protein localizes to discrete foci on the chromosomes of early embryos. These foci correspond to 1.672-and 1.688-g/cm 3 AT-rich satellite repeats found in the centromeric heterochromatin of the X and Y chromosomes and on chromosomes 3 and 4. D1 mRNA levels subsequently decrease throughout later development, followed by the accumulation of the D1 protein in adult gonads, where two distributions of D1 can be correlated to different states of gene activity. We show that the EP473 mutation, a P-element insertion upstream of D1 coding sequences, affects the expression of the D1 gene and results in an embryonic homozygous lethal phenotype correlated with the depletion of D1 protein during embryogenesis. Remarkably, decreased levels of D1 mRNA and protein in heterozygous flies lead to the suppression of position-effect variegation (PEV) of the white gene in the white-mottled (w m4h ) X-chromosome inversion. Our results identify D1 as a DNA-binding protein of known sequence specificity implicated in PEV. D1 is the primary factor that binds the centromeric 1.688-g/cm 3 satellite repeats which are likely involved in whitemottled variegation. We propose that the AT-hook D1 protein nucleates heterochromatin assembly by recruiting specialized transcriptional repressors and/or proteins involved in chromosome condensation.