Gene expression plays a critical role in regulating and modifying various cellular functions, which impact on processes such as development and homeostasis. Gene activation begins in the nucleus, where DNA is transcribed into a nascent transcript that is processed so that non-coding introns are removed by the splicesome, and a cap and poly(A)-tail are added to the beginning and end of the RNA, respectively. Once processing is complete, the mature messenger RNA (mRNA) is exported to the cytoplasm where it localizes to distinct subcellular sites. For example in higher eukaryotes, mRNAs coding for secreted and membrane bound proteins are targeted to the surface of the endoplasmic reticulum (ER).
Our lab utilizes sophisticated cell manipulation protocols, such as nuclear microinjection, in order to figure out:
1) How are mRNAs exported from the nucleus
2) How are mRNAs localized to their proper subcellular destination, such as the endoplasmic reticulum?
We have discovered specialized pathways that promote the export of certain classes of transcripts, those that code for secreted proteins and contain a signal sequence coding region (SSCR), and those that contain a mitochondrial targeting sequence coding region (MSCR). Our data indicates that these RNA elements recruit specialized factors in the nucleus which promote not only export but also the proper translation and localization of mRNA to specialized subcellular sites in the cytoplasm, such as the surface of the endoplasmic reticulum.
Importantly, all RNAs are packaged into larger ribonucleoprotein (RNP) complexes. These complexes may vary considerably between different types of mRNA. The protein component of the RNP dictates where the packaged mRNA is transported, how stable it is, and how efficiently it is translated into protein. We are trying to determine how and where these complexes are assembled and modified throughout the course of an mRNA’s lifetime.
Our work has indicated that messenger RNPs may be modified after they exit the nucleus through the nuclear pore. We have discovered that one nuclear pore protein, RanBP2/Nup358, directly interacts with mRNAs that encode for secretory proteins. Mutations in this gene have been associated with Acute Necrotizing Encephalopathy 1 (ANE1), a rare condition where cytokines are overproduced in response to viral infection. We are currently investigating whether ANE1-associated mutations alter how RanBP2 interacts with cytokine mRNAs.
In other work we have demonstrated that mRNAs coding for secretory and membrane bound proteins can associate with the ER independently of translation. Our results indicate that RNA elements promote the targeting and maintenance of certain transcripts onto the endoplasmic reticulum and that these pathways may act in conjunction with the translation dependent pathway to ensure the fidelity of proper protein sorting to the secretory pathway.
We are now investigating how translation differs between the cytoplasm and different regions of the endoplasmic reticulum, which contains various subdomains including the peripheral tubes, perinuclear sheets, dense tubular matrices and the nuclear envelope.
Our long term goal is to understand how these various mRNA export and localization pathways contribute to cellular organization and gene expression.