Reconstitution of human telomerase with the template RNA component hTR and the catalytic protein subunit hTRT.

The maintenance of chromosome termini, or telomeres, requires the action of the enzyme telomerase, as conventional DNA polymerases cannot fully replicate the ends of linear molecules. Telomerase is expressed and telomere length is maintained in human germ cells and the great majority of primary human tumours. However, telomerase is not detectable in most normal somatic cells; this corresponds to the gradual telomere loss observed with each cell division. It has been proposed that telomere erosion eventually signals entry into senescence or cell crisis and that activation of telomerase is usually required for immortal cell proliferation. In addition to the human telomerase RNA component (hTR; ref. 11), TR1/TLP1 (refs 12, 13), a protein that is homologous to the p80 protein associated with the Tetrahymena enzyme, has been identified in humans. More recently, the human telomerase reverse transcriptase (hTRT; refs 15, 16), which is homologous to the reverse transcriptase (RT)-like proteins associated with the Euplotes aediculatus (Ea_p123), Saccharomyces cerevisiae (Est2p) and Schizosaccharomyces pombe (5pTrt1) telomerases, has been reported to be a telomerase protein subunit. A catalytic function has been demonstrated for Est2p in the RT-like class but not for p80 or its homologues. We now report that in vitro transcription and translation of hTRT when co-synthesized or mixed with hTR reconstitutes telomerase activity that exhibits enzymatic properties like those of the native enzyme. Single amino-acid changes in conserved telomerase-specific and RT motifs reduce or abolish activity, providing direct evidence that hTRT is the catalytic protein component of telomerase. Normal human diploid cells transiently expressing hTRT possessed telomerase activity, demonstrating that hTRT is the limiting component necessary for restoration of telomerase activity in these cells. The ability to reconstitute telomerase permits further analysis of its biochemical and biological roles in cell aging and carcinogenesis.

Aberrant protein phosphorylation at tyrosine is responsible for the growth-inhibitory action of pp60v-src expressed in the yeast Saccharomyces cerevisiae.

Expression of pp60v-src, the transforming protein of Rous sarcoma virus, arrests the growth of the yeast Saccharomyces cerevisiae. To determine the basis of this growth arrest, yeast strains were constructed that expressed either wild-type v-src or various mutant v-src genes under the control of the galactose-inducible, glucose repressible GAL1 promoter. When shifted to galactose medium, cells expressing wild-type v-src ceased growth immediately and lost viability, whereas cells expressing a catalytically inactive mutant (K295M) continued to grow normally, indicating that the kinase activity of pp60v-src is required for its growth inhibitory effect. Mutants of v-src altered in the SH2/SH3 domain (XD4, XD6, SPX1, and SHX13) and a mutant lacking a functional N-terminal myristoylation signal (MM4) caused only a partial inhibition of growth, indicating that complete growth inhibition requires either targeting of the active kinase or binding of the kinase to phosphorylated substrates, or both. Cells arrested by v-src expression displayed aberrant microtubule structures, alterations in DNA content and elevated p34CDC28 kinase activity. Immunoblotting with antiphosphotyrosine antibody showed that many yeast proteins, including the p34CDC28 kinase, became phosphorylated at tyrosine in cells expressing v-src. Both the growth inhibition and the tyrosine-specific protein phosphorylation observed following v-src expression were reversed by co-expression of a mammalian phosphotyrosine-specific phosphoprotein phosphatase (PTP1B). However a v-src mutant with a small insertion in the catalytic domain (SRX5) had the same lethal effect as wild-type v-src, yet induced only very low levels of protein-tyrosine phosphorylation. These results indicate that inappropriate phosphorylation at tyrosine is the primary cause of the lethal effect of pp60v-src expression but suggest that only a limited subset of the phosphorylated proteins are involved in this effect.

The role of the Src homology-2 domain in the lethal effect of Src expression in the yeast Saccharomyces cerevisiae.

Expression of the retroviral transforming gene v-src arrests the proliferation of the yeast Saccharomyces cerevisiae. A functional Src SH2 (Src homology 2) domain is required for this arrest. To examine the mechanism by which Src blocks yeast cell proliferation, and to determine the role of the Src SH2 domain in the growth arrest, src variants were expressed in yeast under the control of the galactose-inducible GAL1 promoter. Following galactose induction of Src expression, phosphotyrosyl-proteins were isolated by immunoprecipitation with beads coupled to either anti-phosphotyrosine antibody or to a recombinant fusion protein containing the Src SH2 domain. A group of SH2-binding phosphotyrosyl proteins was detected in cells expressing toxic forms of Src, but were not detected in cells expressing non-toxic variants. This group of phosphotyrosyl-proteins represents a minor subset of the proteins phosphorylated by v-Src. The lethality of v-Src and the phosphorylation of SH2-binding proteins were co-ordinately affected by alterations in phosphotyrosine-phosphatase activity. These observations indicate that the lethality of Src is correlated with the phosphorylation of proteins that bind to the Src SH2 domain. The role of the SH2 domain in determining the lethal effects of Src in yeast may be similar to its role in targeting Src to substrates necessary for its biological effects in vertebrate cells.

Functional requirement of p23 and Hsp90 in telomerase complexes.

Most normal human diploid cells have no detectable telomerase; however, expression of the catalytic subunit of telomerase is sufficient to induce telomerase activity and, in many cases, will bypass normal senescence. We and others have previously demonstrated in vitro assembly of active telomerase by combining the purified RNA component with the reverse transcriptase catalytic component synthesized in rabbit reticulocyte extract. Here we show that assembly of active telomerase from in vitro-synthesized components requires the contribution of proteins present in reticulocyte extracts. We have identified the molecular chaperones p23 and Hsp90 as proteins that bind to the catalytic subunit of telomerase. Blockade of this interaction inhibits assembly of active telomerase in vitro. Also, a significant fraction of active telomerase from cell extracts is associated with p23 and Hsp90. Consistent with in vitro results, inhibition of Hsp90 function in cells blocks assembly of active telomerase. To our knowledge, p23 and Hsp90 are the first telomerase-associated proteins demonstrated to contribute to telomerase activity.