- Yeast Ash1 mRNA forms a ribonucleoprotein that binds to a myosin motor.
- A motor transports it along actin filaments into the daughter bud.
- It is anchored and translated in the bud, so that the protein is found only in the bud.
An mRNA is synthesized in the nucleus but translated in the cytoplasm of a eukaryotic cell. It passes into the cytoplasm in the form of a ribonucleoprotein particle that is transported through the nuclear pore. Once in the cytosol, it may associate with ribosomes and be translated. The cytosol is a crowded place, occupied by a high concentration of proteins. It is not clear how freely a polysome can diffuse within the cytosol, and most mRNAs are probably translated in random locations, determined by their point of entry into the cytosol, and the distance that they may have moved away from it. However, some mRNAs are translated at specific sites. This may be accomplished by several mechanisms (for review see Palacios and Johnston, 2001; Kloc, Zearfoss, and Etkin, 2002):
- An mRNA may be specifically transported to a site where it is translated.
- It may be universally distributed but degraded at all sites except the site of translation.
- It may be freely diffusible but become trapped at the site of translation.
One of the best characterized cases of localization within a cell is that of Ash1 in yeast (for review see Chartrand, Singer, and Long, 2001). Ash1 represses expression of the HO endonuclease in the budding daughter cell, with the result that HO is expressed only in the mother cell. The consequence is that mating type is changed only in the mother cell (see 18.9 Regulation of HO expression controls switching ). The cause of the restriction to the daughter cell is that all the Ash1 mRNA is transported from the mother cell, where it is made, into the budding daughter cell (Long et al., 1997).
Mutations in any one of 5 genes, called SHE1-5, prevent the specific localization and cause Ash1 mRNA to be symmetrically distributed in both mother and daughter compartments. The proteins She1,2,3 bind Ash1 mRNA into a ribonucleoprotein particle that transports the mRNA into the daughter cell. Figure 5.29 shows the functions of the proteins. She1p is a myosin (previously identified as Myo4), and She3 and She2 are proteins that connect the myosin to the mRNA. The myosin is a motor that moves the mRNA along actin filaments (Bertrand et al., 1998).
Figure 5.30 summarizes the overall process. Ash1 mRNA is exported from the nucleus in the form of a ribonucleoprotein. In the cytoplasm it is first bound by She2, which recognizes some stem-loop secondary structures within the mRNA. Then She3 binds to She2, after which the myosin She1 binds. Then the particle hooks on to an actin filament and moves to the bud. When Ash1 mRNA reaches the bud, it is anchored there, probably by proteins that bind specifically to the mRNA.
Similar principles govern other cases where mRNAs are transported to specific sites. The mRNA is recognized by means of cis-acting sequences, which usually are regions of secondary structure in the 3 untranslated region. (Ash1 mRNA is unusual in that the cis-acting regions are in the coding frame.) The mRNA is packaged into a ribonucleoprotein particle. In some cases, the transported mRNA can be visualized in very large particles, called mRNA granules. The particles are large enough (several times the size of a ribosome) to contain many protein and RNA components (for review see Jansen, 2001).
A transported mRNP must be connected to a motor that moves it along a system of tracks. The tracks can be either actin filaments or microtubules. Whereas Ash1 uses a myosin motor on actin tracks, oscar mRNA in the Drosophila egg uses a kinesin motor to move along microtubules (see 31.7 How are mRNAs and proteins transported and localized?). Once the mRNA reaches its destination, it needs to be anchored in order to prevent it from diffusing away. Less is known about this, but the process appears to be independent of transport. An mRNA that is transported along microtubules may anchored to actin filaments at its destination.