SummaryThe JIL-1 kinase localizes specifically to euchromatin interband regions of polytene chromosomes and is the kinase responsible for histone H3S10 phosphorylation at interphase. Genetic interaction assays with strong JIL-1 hypomorphic loss-of-function alleles have demonstrated that the JIL-1 protein can counterbalance the effect of the major heterochromatin components on position-effect variegation (PEV) and gene silencing. However, it is unclear whether this was a causative effect of the epigenetic H3S10 phosphorylation mark, or whether the effect of the JIL-1 protein on PEV was in fact caused by other functions or structural features of the protein. By transgenically expressing various truncated versions of JIL-1, with or without kinase activity, and assessing their effect on PEV and heterochromatic spreading, we show that the gross perturbation of polytene chromosome morphology observed in JIL-1 null mutants is unrelated to gene silencing in PEV and is likely to occur as a result of faulty polytene chromosome alignment and/or organization, separate from epigenetic regulation of chromatin structure. Furthermore, the findings provide evidence that the epigenetic H3S10 phosphorylation mark itself is necessary for preventing the observed heterochromatic spreading independently of any structural contributions from the JIL-1 protein.Key words: JIL-1 kinase, PEV, Heterochromatin, Gene silencing, Drosophila
IntroductionThe JIL-1 kinase is a multidomain protein that localizes specifically to euchromatin interband regions of polytene chromosomes and is the kinase responsible for histone H3S10 phosphorylation at interphase (Jin et al., 1999;Wang et al., 2001). Mutational analyses have shown that the JIL-1 gene is essential for viability (Wang et al., 2001;Zhang et al., 2003) and that a reduction in JIL-1 kinase activity leads to a global disruption of polytene chromosome morphology (Wang et al., 2001;Deng et al., 2005). Furthermore, genetic interaction assays with JIL-1 hypomorphic and null allelic combinations demonstrated that the JIL-1 protein can counterbalance the effect of the three major heterochromatin components Su(var)3-9, Su(var)3-7 and Su(var)2-5 (HP1a) on position-effect variegation (PEV) (Deng et al., 2010;Wang et al., 2011). Based on these observations, it has been proposed that the epigenetic H3S10 phosphorylation mark functions to counteract heterochromatic spreading and gene silencing in Drosophila melanogaster (Ebert et al., 2004;Zhang et al., 2006;Deng et al., 2007;Deng et al., 2010). However, the previous experiments could not exclude the possibility that the effect of the JIL-1 protein on PEV was instead caused by the gross alterations of polytene chromosome morphology observed in the absence of JIL-1, or arose from structural contributions of the JIL-1 protein, independent of its H3S10 phosphorylation activity. In order to distinguish between these scenarios, we have cloned various full length and truncated versions of the JIL-1 protein into the pYES