No abstract

The apparatus responsible for translocation of proteins across bacterial membranes is the conserved SecY complex, consisting of SecY, SecE, and SecG. Prior genetic analysis provided insight into the mechanisms of protein export, as well as the interactions between the component proteins. In particular, the prl suppressor alleles of secE and secY, which allow export of secretory proteins with defective signal sequences, have proven particularly useful. Here, we report the isolation of novel mutations in secE and secY, as well as the phenotypic effects of combinations of prl mutations. These new alleles, as well as previously characterized prl mutations, were analyzed in light of the recently published crystal structure of the archaeal SecY complex. Our results support and expand a model of Prl suppressor activity that proposes that all of the prlA and prlG alleles either destabilize the closed state of the channel or stabilize the open form. These mutants thus allow channel opening to occur without the triggering event of signal sequence binding that is required in a wild-type complex.

SP/KLF transcription factors comprise an emerging group of proteins that may behave as tumor suppressors. Incidentally, many cancers displaying alterations in certain KLF proteins are also associated with a high incidence of KRAS mutations. Therefore, we here investigate whether SP/KLF proteins suppress KRAS-mediated cell growth, and more importantly, the potential mechanisms underlying these effects. Using a comprehensive, family-wide screening of the 24 SP/KLF members, we discover that SP5, SP8, KLF2, KLF3, KLF4, KLF11, KLF13, KLF14, KLF15 and KLF16 inhibit cellular growth and suppress transformation mediated by oncogenic KRAS. Each protein in this subset of SP/KLF members individually inhibits BrdU incorporation in KRAS oncogenic mutant cancer cells. SP5, KLF3, KLF11, KLF13, KLF14 and KLF16 also increase apoptosis in these cells. Using KLF11 as a representative model for mechanistic studies, we demonstrate that this protein inhibits the ability of cancer cells to form colonies in soft agar and tumor growth in vivo. Molecular studies demonstrate that these effects of KLF11 are mediated, at least in part, through silencing cyclin A via binding to its promoter and leading to cell cycle arrest in S phase. Interestingly, similar to KLF11, KLF14 and KLF16 mechanistically share the ability to modulate the expression of cyclin A. Collectively, this study stringently defines a distinct subset of SP/KLF proteins that impairs KRAS-mediated cell growth, and that mechanistically, some members of this subset accomplish this, at least in part, through regulation of the cyclin A promoter.

Background: Chromatin remodeling mechanisms utilized by Krüppel-like factor proteins remain poorly defined.Results: Krüppel-like factor 11 directly recruits heterochromatin protein 1α to promoters in a sequence-specific manner.Conclusion: Coupling to heterochromatin protein 1 α and its histone methyltransferase underlies Krüppel-like factor-mediated gene expression and tumor suppression.Significance: This data advances our understanding of how chromatin-mediated mechanisms achieve these functions with increased specificity for target genes.