Fission Yeast Int6 Is Not Essential for Global Translation Initiation, but Deletion ofint6⁺Causes Hypersensitivity to Caffeine and Affects Spore Formation

Abstract

Mammalian INT6 protein has been considered to be a subunit of the eukaryotic translation initiation factor, eIF3. The Int6 locus is also known as a common integration site of mouse mammary tumor virus (MMTV). However, the function of Int6 in translation initiation and the mechanism of Int6-mediated tumor induction are yet to be explored. In this study, the fission yeast, Schizosaccharomyces pombe, int6(+), which is 43% identical to the mammalian counterpart, was deleted. Despite the evidence that the majority of Int6 protein was associated with 40S particles in this organism, strains lacking int6(+) (Deltaint6) were viable and showed only moderate inhibition in the rate of in vivo global protein synthesis. Polysome profile analysis showed no apparent defects in translation initiation. Deltaint6 exhibited a hypersensitivity to caffeine, which could be suppressed by the addition of sorbitol to the growth medium. This and other phenotypes would imply that int6(+) is required for the integrity of cell membrane. In meiosis, Deltaint6 produced incomplete tetrads frequently. High dosage expression of a truncated mutant of int6(+) conferred a hypersensitivity to caffeine, but did not cause the defect in meiosis. A possible link between the function of int6(+) and the Deltaint6-phenotypes is discussed.

An oncogenic role of eIF3e/INT6 in human breast cancer

“…This could reflect a direct interaction between the two complexes, or alternatively an influence of eIF3 on other events required for 60S joining, such as GTP hydrolysis by eIF2 (Merrick, 1979). In agreement with our results, fission yeast strains lacking eIF3e are viable, and polysome profile analysis showed that they have no major defects in translation initiation (Bandyopadhyay et al, 2000;Crane et al, 2000), though a decreased level of de novo protein synthesis was detected by pulse labeling in a more recent study (Udagawa et al, 2008). Fission yeast eIF3e and eIF3m define two distinct eIF3 complexes that may promote the translation of different sets of mRNAs; the eIF3m complex associated with bulk cellular mRNAs, whereas the eIF3e-containing complex associated with a far more restricted set (Zhou et al, 2005).…”

Silencing of human Int-6 impairs mitosis progression and inhibits cyclin B–Cdk1 activation

“…Conversely, overexpression of Schizosaccharomyces pombe Int-6 causes resistance to caffeine, similar to what has been observed for the Poh1 (Rpn11) subunit of the proteasome (Spataro et al, 1997;Crane et al, 2000). It has also been shown that depletion of Int-6 in S. pombe affects spore formation and causes weak chromosome segregation defects (Bandyopadhyay et al, 2000;Yen and Chang, 2000). More recently, Yen et al have shown that combination of Int-6 depletion with proteasome subunit mutants leads to a blockade of fission yeast division at the metaphase to anaphase transition.…”

“…This event weakly modifies general translation, but induces sensitivity to various drugs such as caffeine (Bandyopadhyay et al, 2000;Akiyoshi et al, 2001). Conversely, overexpression of Schizosaccharomyces pombe Int-6 causes resistance to caffeine, similar to what has been observed for the Poh1 (Rpn11) subunit of the proteasome (Spataro et al, 1997;Crane et al, 2000).…”

Evidence for the transforming activity of a truncated Int6 gene, in vitro

“…An Int6 homolog was found in ®ssion yeast (Bandyopadhyay et al, 2000;Crane et al, 2000). The yeast Int6 protein was found strongly associated with 40S ribosomal particles suggesting a common role with mammalian Int6 in translation initiation.…”