Although these complexes are easy to observe in the fluorescence microscope, attempts to elucidate their detailed composition and structure have failed so far. "We chose baker's yeast, Saccharomyces cerevisiae, as a model, because it has a simple system for mRNA transport," says Dr. Marisa Mller, who performed a large body of the experiments for the new study. "Only a small number of factors are involved and all have already been identified."
"We know that, in addition to the motor protein Myo4p, factors called She2p and She3p are required for mRNA transport activity," adds Roland Heym, who shares first-author status with Mller. She2p is known to bind to RNA, and was thought to be the only factor required for the recognition of the mRNA to be transported. This was expected to be a very early step, taking place in the nucleus, possibly immediately after transcription of the mRNA from the genomic DNA.
The transport phase itself begins in the cytoplasm, i.e. after the mRNA has been exported from the nucleus. This is where She3p comes into play. She3p acts as an adaptor that forms a bridge between She2p (with its associated mRNA cargo) and the motor protein Myo4p. Using a combination of biochemical, biophysical and in-vivo imaging methods, Niessing and his team were able to work out how, when and in what order the transport complex is assembled.
"To our surprise, we found that She2p forms stable and specific complexes with mRNA only in the presence of She3p, and then only with mRNAs that are destined to be transported," says Niessing. "Moreover, She3p not only interacts with She2p and the motor Myo4p, but forms direct contacts with the mRNA. mRNA binding by the indi
|Contact: Luise Dirscherl|