The LINE-1 (L1) family of non-long terminal repeat retrotransposons is a major force shaping mammalian genomes, and its members can alter the genome in many ways. Mutational analyses have shown that coexpression of functional proteins encoded by the two L1-specific open reading frames, ORF1 and ORF2, is an essential prerequisite for the propagation of L1 elements in the genome. However, all efforts to identify ORF2-encoded proteins have failed so far. Here, applying a novel antibody we report the presence of proteins encoded by ORF2 in a subset of cellular components of human male gonads. Immunohistochemical analyses revealed coexpression of ORF1 and ORF2 in prespermatogonia of fetal testis, in germ cells of adult testis, and in distinct somatic cell types, such as Leydig, Sertoli, and vascular endothelial cells. Coexpression of both proteins in male germ cells is necessary for the observed genomic expansion of the number of L1 elements. Peptide mass fingerprinting analysis of a ϳ130-kDa polypeptide isolated from cultured human dermal microvascular endothelial cells led to the identification of ORF2-encoded peptides. An isolated ϳ45-kDa polypeptide was shown to derive from nonfunctional copies of ORF2 coding regions. The presence of both ORF1-and ORF2-encoded proteins in vascular endothelial cells and its apparent association with certain stages of differentiation and maturation of blood vessels may have functional relevance for vasculogenesis and/or angiogenesis. LINE-1 (long interspersed nuclear element) retrotransposons (L1)1 cover about 17% of the human genome (1) and play a significant role in shaping the mammalian genome, not only through their own expansion but also through the mobilization of non-L1 sequences. Although the average haploid human genome harbors ϳ516,000 L1 copies, the subgroup of active L1s is fairly small, encompassing 80 -100 elements (2, 3). So far, 82 retrotransposition-competent, full-length L1 elements were isolated and characterized (2-6). Remarkably, 84% of the retrotransposition capability of those elements was shown to be present in six highly active L1s (3). L1s affected the genome by (i) insertion of truncated L1s into new sites, (ii) intrachromosomal homologous recombination between L1s, (iii) transduction of 3Ј-flanking sequences during retrotransposition, (iv) aiding trans generation of processed pseudogenes and retrotransposition of Alu elements (7), and (v) by causing genome instability through substantial deletions (8, 9). A retrotransposition-competent, functional L1 element (RC-L1) covers ϳ6.1 kb and contains a 5Ј-untranslated region with an internal, CpG-rich promoter, a 1-kb ORF1 encoding a protein (p40) of approximately 40 kDa with RNA binding capability, followed by a 3.8-kb ORF2 coding for a protein (p150) with a predicted molecular mass of about 150 kDa with endonuclease (EN) and reverse transcription (RT) activities and a cysteine-histidine-rich domain. The 3Ј-end of L1 is terminated by a short 3Ј-untranslated region and a poly(A) tail (10) (Fig. 1A). L1 mRNAs ar...

LINE-1 (L1) is a highly successful autonomous non-LTR retrotransposon and a major force shaping mammalian genomes. Although there are about 600 000 L1 copies covering 23% of the rat genome, full-length rat L1s (L1Rn) with intact open reading frames (ORFs) representing functional master copies for retrotransposition have not been identified yet. In conjunction with studies to elucidate the role of L1 retrotransposons in tumorigenesis, we isolated and characterized 10 different cDNAs from transcribed full-length L1Rn elements in rat chloroleukemia (RCL) cells, each encoding intact ORF1 proteins (ORF1p). We identified the first functional L1Rn retrotransposon from this pool of cDNAs, determined its activity in HeLa cells and in the RCL cell line the cDNAs originated from and demonstrate that it is mobilized in the tumor cell line in which it is expressed. Furthermore, we generated monoclonal antibodies directed against L1Rn ORF1 and ORF2-encoded recombinant proteins, analyzed the expression of L1-encoded proteins and found ORF1p predominantly in the nucleus. Our results support the hypothesis that the reported explosive amplification of genomic L1Rn sequences after their transcriptional activation in RCL cells is based on L1 retrotransposition. Therefore, L1 activity might be one cause for genomic instability observed during the progression of leukemia.