Ikaros proteins are required for normal T, B, and NK cell development and are postulated to activate lymphocyte-specific gene expression. Here we examined Ikaros distribution in the nucleus of B lymphocytes using confocal microscopy and a novel immunofluorescence in situ hybridization (immuno-FISH) approach. Unexpectedly, Ikaros localized to discrete heterochromatin-containing foci in interphase nuclei, which comprise clusters of centromeric DNA as defined by gamma-satellite sequences and the abundance of heterochromatin protein-1 (HP-1). Using locus-specific probes for CD2, CD4, CD8alpha, CD19, CD45, and lambda5 genes, we show that transcriptionally inactive but not transcriptionally active genes associate with Ikaros-heterochromatin foci. These findings support a model of organization of the nucleus in which repressed genes are selectively recruited into centromeric domains.

Stathmin is a cytosolic phosphoprotein that has an important but, as yet, undefined role in cell proliferation and differentiation. Induction of growth arrest and differentiation of HL60 cells to monocytes by phorbol 12-myristate 13-acetate is associated with rapid phosphorylation of the protein. Stathmin phosphorylation was not seen when HL60 cells were induced to differentiate to monocytes, by 1 alpha, 25-dihydroxyvitamin D3, and to neutrophils, by all-trans retinoic acid and granulocyte colony stimulating factor. In all the above instances, stathmin expression was down-regulated. Thus, increased stathmin phosphorylation is not required for cell growth arrest or differentiation or down-regulation of stathmin expression.

It is known that on phage RNda there i e only one 'antry'eite for the ribosomethe h t iation region of the ooat protein g e m . 3 S ri-b0EiOlnd Bubunit6 are able ize this site even in the absenoetzfrEgtor tRNd, but protein S l appear6 to be strongly needed for the reooepletion prooees.. We have investigated where S l binde to phage UAEI within 305 preinitiation oomplexes formed in t h e abeenoe of initiator tRNA. Resate o r sequenoing allow us to determine w i t h i n the phage RHA primarg atruorure the lomtion of S1-bound fragmente. All the f , the major and minor O r U 3 6 , b e l O n g to aZ:'region of phage RNA eequenoe rshioh ie looated upstream from the ooat protein gene:h the oaBe of QB RNB this re ion oomprieea nuoleotides 1247-1322. with fhe ooat protein intiation oodon being at 1344; Fn the 0888 of fr RNA nuoleotides 1226-1297 and 1336. oorrespondinglg. The region proteotid by oovalently bound S1 againet de iation with RNam Ti oompletelg overlape 8 t h the so oalled 5-site Q$ r e p l i o m e binding. S1 ia known to be a tramlstional repreasor or the ooat protein eintheeie UCtTO Prom our data this effeot a m be explained a6 ampetition between free S1 and S1 as a part of the 305 subunit for the aame on the messenger. We propose that the aitea oontaining oligo(U) sequenoea in Qp and Ir RNda ,serve as reoo tion signals r o r 305 riboeomee during ini%tion oomplex formation.The p21 CiplIWafl Cyclin dependent kinase inhibitor has been implicated in G1 arrest following irradiation,as well as in maintaining terminally arrested,differentiated cells in GO/Gl.The G1 arrest is mediated by inhibiting the Cdk dependent phosphorylation of the retinoblastoma protein,Rb.We sought to understand better these effects,and additionally what effects p21 might have in S phase and G2/M,by expressing p21 in a panel of human cell lines using a tetracycline controlled inducible system,as well as using an adenoviral transduction vector.Our results concerning our observations of differential cell cycle arrest,apoptosis,and the underlying biochemical mechanisms of these effects will be presented. Stathmin is a prominent cytosolic protein which can be phosphorylated by MAP kinase, CAMP-dependent protein kinase and p34& kinase. Immunostaining of human and mouse tissues has revealed that stathmin is expressed in the proliferative compartment of cells of most, if not all, cell lineages. The myeloid cell liaes HL60 and K562 express stathmin at very high levels. When these cells are induced to differentiate, stathmin expression is down-regulated as cells go out of cycle. A decrease in the proportion of stathmin that is phosphorylated was also observed. To investigate the role that stathmin plays in the control of cell proliferation, we have transfected the promonoqtoid cell line U937 with the pMep4 vector containing stathmin cDNA in an antisense orientation. Antisense stathmin rransfected cells were unable to undergo normal cell division and became multinucleate. Belmont and Mitchison (Cell 1996, 84, 623431) have shown in Virro that stathmi...