Telomeres of human chromosomes consist of a repeated TTAGGG sequence, and at the terminal of this repeat sequence, the 3' strand is longer than the 5' strand. In this study, we characterized single and double stranded telomere binding proteins (ssTBPs and dsTBPs) by gel mobility shift assay and South-Western blotting assay. At least two protein components with molecular weights of 29 and 33 kDa were bound to a single stranded telomeric sequence, and also two proteins with molecular weights of about 44 kDa and 70 kDa were bound to a double stranded telomeric sequence. A competition assay demonstrated that the binding properties of ssTBPs and dsTBPs were specific to the telomeric sequence. We further cloned a ssTBP cDNA (ssTBP-1) by screening a lambda-gt11 expression library and identified ssTBP-1 as a human heterogeneous nuclear ribonucleoprotein (hnRNP) Al on the basis of cDNA sequence. We also found that the expression of the hnRNP Al gene significantly decreased during in vitro passage of human microvascular endothelial cells.

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