mRNA capping enzyme exports to cytoplasm, localizes to stress granules and maintains cap homeostasis of the target mRNAs

mRNA decapping is believed to trigger RNA degradation until the identification of cytoplasmic capping that has changed the epitome of RNA stability. Unlike nuclear capping machinery that includes RNA polymerase II bound mRNA Capping Enzyme (CE), N-7 RNA methyl transferase and RNMT activating protein RAM, cytoplasmic capping complex consist of cytoplasmic pool of CE (cCE) and N-7 RNA methyl transferase-RAM along with a few cytoplasmic proteins of various functions. Cytoplasmic capping has been shown to recap selective uncapped mRNAs and maintains cap homeostasis by a cyclic process of decapping and recapping. Thus, it acts as post-transcriptional nexus for the target transcripts. Our data show nuclear export of mammalian CE is regulated by Exportin1 (XPO1) pathway via a conserved Nuclear Export Signal sequence. In order to examine biological function of cCE, we show cCE forms granules during stress and majority of these granules co-localize with SGs. In order to identify how cCE regulates cap homeostasis during stress and recovery, we measured the cap status of specific cCE targeted mRNA transcripts along with non-targeted transcripts during non-stress, stress and recovery phase using Xrm1 susceptibility assay. Our data show cCE targeted mRNA transcripts lost their caps in stress condition when cCE is sequestered in granules. After removal of stress, when cCE is released, the cap status has been restored for these transcripts pointing towards the role of cCE in altering cap homeostasis and thus promoting cellular recovery from stress.