Protein stabilization: a common consequence of mutations in independently derived v-Myc alleles

Abstract

Myc is overexpressed in many cancers as a result of gene rearrangement or ampli®cation, but coding sequence changes which cluster in the N-terminal transactivation domain also appear to play a role in tumour progression. The prototypic v-Myc gene of MC29 virus diers from avian c-Myc by a series of mutations, including a change at a regulatory phosphorylation site within the mutational hotspot (thr-61) which is known to potentiate transformation in vitro. We now show that the mutation at thr-61 stabilizes the v-Myc protein (turnover dierence) and that this single mutation is both necessary and sucient for the phenotype. A major involvement of the proteasome in Myc degradation was con®rmed, but surprisingly, a dilysine motif adjacent to thr-61 proved not to be the ubiquitin target. Two other v-Myc genes which carry a mutation at thr-61 (avian MH2) or a large deletion encompassing this domain (feline T17) were found to be stabilized to a similar extent as MC29, showing that stabilization is a common feature of independently derived Myc oncogenes. These results suggest a common selective process in the genesis of these three viral oncoproteins and a mechanistic link with Jun, Fos and Myb oncoproteins which are also stabilized relative to their cellular counterparts.

Transforming ability of Gag-Myc fusion proteins correlates with Gag-Myc protein stability and transcriptional repression

“…Our data con®rm results of others that increased Myc stability in the cell is related to transforming ability (Gavine et al, 1999;Salghetti et al, 1999). However, we found that the di erences in Gag-Myc protein half-lives were not re¯ected in the steady state levels ( Figure 6).…”

“…v-Myc was shown to be three times more stable than c-Myc, but with a 61Met-4Thr backmutation, v-Myc was degraded in the same time frame as c-Myc with a half life of 20 ± 30 min (Gavine et al, 1999;Hann and Eisenman, 1984). v-Mycs from MH2 and T17, a feline myc-transducing retrovirus, were also more stable than c-Myc (Gavine et al, 1999).…”

“…v-Myc was shown to be three times more stable than c-Myc, but with a 61Met-4Thr backmutation, v-Myc was degraded in the same time frame as c-Myc with a half life of 20 ± 30 min (Gavine et al, 1999;Hann and Eisenman, 1984). v-Mycs from MH2 and T17, a feline myc-transducing retrovirus, were also more stable than c-Myc (Gavine et al, 1999). Other studies have shown that Myc proteins isolated from lymphoma, glioma, and neuroblastoma cell lines and from lymphoma patient samples have longer half lives than c-Myc, in some cases as much as 110 min (Cohn et al, 1990;Salghetti et al, 1999;Shindo et al, 1993).…”

“…Myc proteins that are isolated from some cancer patients have previously been shown to be more stable than c-Myc (Cohn et al, 1990;Salghetti et al, 1999;Shindo et al, 1993) as are v-Myc proteins with the amino acid 61 Thr-4Met mutation (Gavine et al, 1999). We performed pulse-chase experiments to determine the half-lives of the Gag-Myc proteins used in this study.…”

“…In addition to protein overexpression, v-Myc proteins and Myc proteins isolated from cancer cell lines and cancer patient samples have been shown to exhibit longer half-lives than their normal cellular counterparts which may contribute to a sustained e ect on Myc gene targets (Gavine et al, 1999;Salghetti et al, 1999). v-Myc was shown to be three times more stable than c-Myc, but with a 61Met-4Thr backmutation, v-Myc was degraded in the same time frame as c-Myc with a half life of 20 ± 30 min (Gavine et al, 1999;Hann and Eisenman, 1984).…”