Most studies on the structure of DNA in telomeres have been dedicated to the double-stranded region or the guanosine-rich strand and consequently little is known about the factors that may bind to the telomere cytosine-rich (C-rich) strand. This led us to investigate whether proteins exist that can recognise C-rich sequences. We have isolated several nuclear factors from human cell extracts that specifically bind the C-rich strand of vertebrate telomeres [namely a d(CCCTAA)(n)repeat] with high affinity and bind double-stranded telomeric DNA with a 100xreduced affinity. A biochemical assay allowed us to characterise four proteins of apparent molecular weights 66-64, 45 and 35 kDa, respectively. To identify these polypeptides we screened alambdagt11-based cDNA expression library, obtained from human HeLa cells using a radiolabelled telomeric oligonucleotide as a probe. Two clones were purified and sequenced: the first corresponded to the hnRNP K protein and the second to the ASF/SF2 splicing factor. Confirmation of the screening results was obtained with recombinant proteins, both of which bind to the human telomeric C-rich strand in vitro.

Signal regulatory proteins of the ␣ subtype (SIRP␣) are ubiquitous molecules of the immunoglobulin superfamily that negatively regulate protein tyrosine kinase receptor-dependent cell proliferation. Their intracytoplasmic domain contains four motifs that resemble immunoreceptor tyrosine-based inhibition motifs (ITIMs) and that, when tyrosyl-phosphorylated, recruit cytoplasmic SH2 domain-bearing protein tyrosine phosphatases (SHPs). ITIMs are borne by molecules that negatively regulate cell activation induced by receptors bearing immunoreceptor tyrosine-based activation motifs (ITAMs). Because SIRP␣ are coexpressed with ITAM-bearing receptors in hematopoietic cells, we investigated whether SIRP␣ could negatively regulate ITAM-dependent cell activation. We found SIRP␣ transcripts in human mast cells, and we show that a chimeric molecule having the transmembrane and intracytoplasmic domains of SIRP␣ could inhibit IgE-induced mediator secretion and cytokine synthesis by mast cells. Inhibition required that the SIRP␣ chimera was coaggregated with ITAM-bearing high affinity IgE receptors (Fc⑀RI). It was correlated with the tyrosyl phosphorylation of the SIRP␣ chimera and the recruitment of SHP-1 and SHP-2. The phosphorylation of Fc⑀RI ITAMs was decreased; the mobilization of intracellular Ca 2؉ and the influx of extracellular Ca 2؉ were reduced, and the activation of the mitogen-activated protein kinases Erk1 and Erk2 was abolished. SIRP␣ can therefore negatively regulate not only receptor tyrosine kinase-dependent cell proliferation but also ITAM-dependent cell activation.Signal regulatory proteins (SIRPs) 1 were described as phosphoproteins that coprecipitated with SH2 domain-bearing protein tyrosine phosphatases (SHPs) (1-3). SIRP molecules were first identified as SHP substrate 1 (SHPS-1) (2) and brain immunoglobulin-like molecules with tyrosine-based activation motifs (BIT) (4). When cloned, these molecules were found to be the members of a multigene family of at least 15 transmembrane immunoglobulin superfamily molecules named collectively SIRPs, in which two types, ␣ and ␤, were recognized, differing by the presence (in SIRP␣) or the absence (in SIRP␤) of an intracytoplasmic (IC) domain containing four tyrosinebased regulatory motifs (3). A neuronal adhesion molecule, previously described as P84, was found to belong to the SIRP family, and the widely expressed integrin-associated protein CD47 was recently identified as a ligand of P84 (5). Interestingly, SIRP␣ were shown to regulate negatively cell proliferation induced by growth factors via protein tyrosine kinase receptors (RTKs) and oncogene products. Little is known of the mechanism of inhibition by SIRP␣, except that negative regulation was correlated with the tyrosyl phosphorylation of SIRP␣ and the recruitment of SHP-2 (3, 6). That SIRP␣ had inhibitory properties indicated that these molecules bear inhibition motifs rather than activation motifs. SHP-2-binding motifs found in SIRP␣ are indeed reminiscent of immunoreceptor tyrosine-based inhibition motifs...