Small ubiquitin-like modifier conjugation regulates nuclear export of TEL, a putative tumor suppressor

Abstract

Posttranslational modification by small ubiquitin-like modifier (SUMO) conjugation regulates the subnuclear localization of several proteins; however, SUMO modification has not been directly linked to nuclear export. The ETS (E-Twenty-Six) family member TEL (ETV6) is a transcriptional repressor that can inhibit Rasdependent colony growth in soft agar and induce cellular aggregation of Ras-transformed cells. TEL is frequently disrupted by chromosomal translocations such as the t(12;21), which is associated with nearly one-fourth of pediatric B cell acute lymphoblastic leukemia. In the vast majority of t(12;21)-containing cases, the second allele of TEL is deleted, suggesting that inactivation of TEL contributes to the disease. Although TEL functions in the nucleus as a DNA-binding transcriptional repressor, it has also been detected in the cytoplasm. Here we demonstrate that TEL is actively exported from the nucleus in a leptomycin B-sensitive manner. TEL is posttranslationally modified by sumoylation at lysine 99 within a highly conserved domain (the ''pointed'' domain). Mutation of the sumo-acceptor lysine or mutations within the pointed domain that affect sumoylation impair nuclear export of TEL. Mutation of lysine 99 also results in an increase in TEL transcriptional repression, presumably because of decreased nuclear export. We propose that the ability of TEL to repress transcription and suppress growth is regulated by sumoylation and nuclear export.T he function of posttranslational modification by ligation of a small ubiquitin-like modifier (SUMO) to a target protein appears to be diverse and substrate specific. RanGAP1 was the first protein shown to be modified by addition of SUMO (1-3). Unmodified RanGAP1 is diffusely cytoplasmic, whereas sumoylation targets RanGAP1 to nuclear pore complexes. Sumoylation also directs targeted proteins such as promyelocytic leukemia and the homeodomain-interacting protein kinase 2 to distinct subnuclear domains (nuclear bodies or speckles; refs. 4 and 5). SUMO modification appears to activate the heat shock transcription factors 1 and 2 (6, 7), whereas sumoylation may negatively regulate c-JUN and c-MYB activity (8, 9). Sumoylation also affects protein stability if the modified lysine is also used for ubiquitination. Both inhibitor of NF-B␣ and murine double minute 2 are targeted for degradation by ubiquitination, but competition for the target lysine by SUMO stabilized these factors (10-12). Like ubiquitin, SUMO conjugation and deconjugation is a very dynamic process. Sumoylation studies are limited because of the rapid removal of the modification by SUMO-specific isopeptidases in cellular extracts. Thus, for each of these examples, the functional consequences of sumoylation were derived from conservative substitution of the modified lysine of the target protein.Like ubiquitination, sumoylation is a three-step process involving an E1-activating enzyme heterodimer Aos͞Uba2, the E2-conjugating enzyme Ubc9 and substrate-specific E3 ligases (for recent reviews, see refs. 13 ...

Somatic heterozygous mutations in ETV6 (TEL) and frequent absence of ETV6 protein in acute myeloid leukemia

“…The C-terminal ETV6 mutations were either inframe or resulted in truncation of the ETS domain by out-of-frame mutation. In concordance with earlier studies (Waga et al, 2003;Wood et al, 2003), the truncated ETV6 proteins were impaired in repressing transcription in a ETV6-specific luciferase assay. Moreover, coexpression of the mutants with an intact homodimerization domain (#2236, 3102, 2649 and N) completely abolished the transcription repression by wt ETV6.…”

SUMO: a regulator of gene expression and genome integrity

“…Work in Dictyostelium on the MAP kinase MEK-1 revealed that SUMOylation is required for MEK1's translocation from the nucleus to the cytosol (Galy et al, 2002). A similar mechanism was proposed recently for the transcriptional repressor TEL (Wood et al, 2003). The localization of a SUMO-ligase at the nuclear pore now strengthens the idea that SUMOylation is coupled to nuclear transport processes.…”

Medea SUMOylation restricts the signaling range of the Dpp morphogen in the Drosophila embryo

“…For example, SUMO has been implicated in promoting the nuclear retention of the Elk-1 transcription factor (Salinas et al 2004), adenoviral E1B-55K protein (Kindsmuller et al 2007), and CtBP1 corepressor (Lin et al 2003b). In terms of SUMO promoting nuclear export, as our data suggest for Med, examples include the TEL repressor protein (Wood et al 2003), MEK1 kinase (Sobko et al 2002), ribosome biogenesis factors (Panse et al 2006), and p53 transcription factor (Carter et al 2007).…”