In a multicellular organism, different cells fulfill a range of diversified functions. Often such specialization depends on the delivery of molecular goods to distinct places within a cell. It ensures that particular functions only occur at defined cellular sites. This establishment of intracellular asymmetry in the otherwise fluid environment of the cell cytoplasm requires active transport processes. Messenger RNAs (mRNA) represent an especially important type of freight. They are copies of genetic information stored in the nucleus. In the cytoplasm the information encoded in mRNAs is used for the synthesis of proteins.
Obviously it makes sense to manufacture certain proteins at their future site of action. "Unfortunately, we know very little about the molecular basis of this freight system," says Dr. Dierk Niessing, who heads a research group affiliated with the Helmholtz Zentrum München at LMU's Gene Center. "We have now deciphered how one of these transport complexes from yeast cells recognizes its cargo mRNA and initiates assembly." The new findings might also be applicable to higher organisms, where transport processes are especially critical for cell function. For instance, the activity and plasticity of synapses -- the interfaces between neurons that are responsible for the transmission of nerve impulses -- is dependent on the transport of specific mRNAs from the nucleus along the nerve fibers. In cases where localization is essential, the misdirection of goods causes chaos and result in cell death.
All cells containing a nucleus also possess a cytoskeleton made up of filamentous protein strands that course through intracellular space like a rail network. Motor proteins "walk" along two types of these strands, called actin filaments and microtubules. On their way, they can carry different types of freight like membrane vesicles, messenger RNAs, proteins and even whole organelles. Disruption of these networks can cause chaos and may result in cell death. The motor proteins must recognize and bind to the correct fiber system and to the appropriate cargo. For transport of freight, molecular motors interact with a plethora of accessory factors to form large transport complexes.
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 Müller, 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 Müller. 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 individual proteins is not sufficient for selection of transcripts for transport. She2p, She3p and mRNA must come together to permit formation of a stable and specific complex."
Moreover, the two proteins check the passenger tickets, so to speak, because the complex remains stable only if the mRNA carries a particular molecular label that earmarks it as cargo. If the label is missing, the complex rapidly disintegrates. "Our results demonstrate yet again how little we know about this essential cellular process," says Niessing. "It will take a whole series of analogous experiments to understand the mechanistic similarities and differences between the systems that mediate molecular transport in simple unicellular and sophisticated multicellular organisms."
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