Peripherin particles often contain multiple mRNAs (number indicated by colors at left).

Findings on page 747 suggest that multiple subunits of coiled-coil proteins may meet even before each subunit is fully translated. The article from Chang et al. reveals that the precursors to intermediate filaments (IFs) assemble at tiny translation factories containing several mRNAs.

Dimers of IFs, such as the neuronal protein peripherin, form into higher-order assemblies. Before they form filaments, insoluble aggregates of dimers or larger nonfilamentous structures can be visualized as cytoplasmic particles. The new results show that many of these particles are associated with peripherin mRNA.

The authors directly visualized peripherin mRNA and protein product simultaneously in living cells. The peripherin mRNA and protein particles seemed to move independently along microtubules. Stationary mRNAs, however, were also seen associated with protein particles, suggesting that only immobile mRNAs are translating. Once translation finished, the ribosomes appeared to dissociate. The mRNAs then moved rapidly away from the newly synthesized protein particle, possibly for degradation or translation elsewhere.

The vast majority (80%) of peripherin mRNA was found in clusters of two or more mRNAs. Several features seem to contribute to their clustering: in addition to microtubules, the transcript's 3′ UTR was partly required, as was ongoing translation, suggesting that both mRNA sequences and interactions between nascent peripherin chains help to bring the mRNAs together.

Transcripts of another IF, keratin, were also found in clusters. For both peripherin and keratin, these mRNA clusters may reflect the coordinated translation of protein chains from closely aligned mRNAs, thus allowing for efficient cotranslational assembly of coiled-coil proteins. Intermediates of myosin, which is another coiled-coil protein, have also been seen in globular foci that form cotranslationally during muscle maturation.