Cross-border traffic: A protein linking two steps of gene expression

Gene expression is a fundamental biological process by which the genetic information is converted into proteins - a complicated process consisting of many different steps. It begins with the transcription (copying) of defined sections of the DNA (genes) into a related molecule called messenger RNA (mRNA). This mRNA then undergoes several processing steps before it is delivered to the so called ribosomes that translate the base sequence of the mRNA into a protein - a sequence of amino acids. Higher organisms like ourselves are referred to as eukaryotes, because in our cells (unlike bacterial cells) the DNA resides in a specialized compartment -- the nucleus, while protein synthesis occurs in the cytoplasm that surrounds it. Thus, our mRNAs must be exported from the nucleus to the cytoplasm before they can be translated. LMU biologist Dr. Katja Straber studies how the different steps of gene expression are linked, and how this coupling contributes to the efficiency of the whole process. Together with members of her research group, she has now shown that an RNA-binding protein called Sro9 links transcription to translation. "Sro9 shuttles between nucleus and cytoplasm", says Straber. "We suspect that the protein binds to the newly synthesized mRNA during transcription in the nucleus and is exported together with the mRNA to the cytoplasm where it is required for efficient protein synthesis. Thus, Sro9 might functionally couple transcription and translation, which could increase the efficiency of gene expression and also allow for quality control." (RNA online, 21. Mai 2010)

All living cells contain double-stranded DNA molecules that reside within the nucleus in eukaryotic cells and encode thousands of genes. Genes specify the sequences of amino acids that make up proteins, and proteins execute the tasks that each cell requires for its viability and specialized functions. The process of converting a particular DNA sequence into a protein with a defined function involves a whole series of events, each of which depends on successful completion of the preceding steps. The first step involves the transcription of a definite segment of one strand of the DNA into a single-stranded RNA molecule called mRNA. The mRNA is first processed before it is exported from the nucleus to the cytoplasm. In the cytoplasm the mRNA sequence is translated into the amino acid sequence of the encoded protein by specialized machines called ribosomes. Proteins are responsible for the structure, function and behavior of the cell, acting for example as enzymes catalyzing chemical reactions or as transporters of biomolecules.

"The process of gene expression is fundamental to all forms of life, but its two major steps, transcription and translation, have generally been viewed as not only spatially but also functionally separate processes", Dr. Straber points out. It is now clear, however, that, at all stages of gene expression, the mRNA is associated with many different proteins. It is thought that these mRNA-binding proteins play specific roles in the regulation of successive steps in gene expression.

The protein Sro9 was already known to act in transcription, translation, and stabilization of mRNAs. Straber and her coworkers now show that Sro9 is recruited to actively transcribed genes, and shuttles between nucleus and cytoplasm. "We postulate that the protein already binds to the growing mRNA chain during its synthesis, making transcription more efficient", says Straber. "Sro9 is then exported from the nucleus together with the mRNA to act in translation. Sro9 probably belongs to a growing set of mRNA binding proteins that link nuclear and cytoplasmic steps of gene expression. The molecular coupling of different steps in gene expression not only enhances the efficiency of this process, it also offers a means of quality control." (suwe)

This work was supported by the Cluster of Excellence "Center for Integrated Protein Science Munich" (CIPSM), the Priority Program (SFB) 646 "Regulatory Networks in Genome Expression and Maintenance", and an ERC Starting Grant of the EU.

Publication:
"Nucleocytoplasmic shuttling of the La-motif containing protein Sro9 might link its nuclear and cytoplasmic functions",
Susanne Rother, Cornelia Burkert, Katharina M. Brunger, Andreas Mayer, Anja Kieser, and Katja Straber
RNA online, 21. Mai 2010
DOI: 10.1261/rna.2089110

Contact: Dr. Katja Straber
Gene Center Munich
Phone: +49 (0) 89 / 2180 – 76937v Email: strasser@lmb.uni-muenchen.de