One of the first discoveries of gene expression mediated by controlling messenger RNA (mRNA) stability is autoregulation of tubulin synthesis. In this regulatory process, the concentration of tubulin subunits modulates the stability of the mRNAs from which they are translated (12). In the 1980s it was found that only translated tubulin mRNAs are autoregulated (3) and that translation had to continue through at least 41 amino acids (4). This is enough for the nascent tubulin polypeptide to emerge from the ribosome (5). Later work established that when the tubulin subunit pool is high, the first four amino acids (Met-Arg-Glu-Ile, MREI) emerging during nascent tubulin translation serve as a regulatory tag. Recognition of this tag promoted the degradation of the translating tubulin mRNA (468). More than 30 years later, on page 100 of this issue, Lin et al. (9) identify tetratricopeptide repeat protein 5 (TTC5) as the regulator that binds to nascent tubulin polypeptides.

With the exception of a high-resolution confirmation that an elevated pool of tubulin subunits selectively represses tubulin synthesis (8), no progress toward understanding the autoregulation of tubulin expression was made since 1988. The most attractive model for how the pool size of tubulin subunits could trigger rapid mRNA degradation to suppress new tubulin synthesis was that it was the tubulin α/β dimer, the unit that assembles into microtubules, that cotranslationally bound to the MREI tetrapeptide. Lin et al. disprove this model. They use mass spectrometry and in vitro translation of an mRNA encoding the first 94 amino acids of β-tubulin to identify TTC5 as the protein that recognizes the nascent β-tubulin MREI tetrapeptide in complex with the large ribosomal subunit.

α- and β-tubulin form a heterodimer that serves as the building block for microtubule polymers, the tracks along which cargoes are moved by dynein and kinesin family motors. During cell duplication, microtubule-directed trafficking is essential for delivery to each daughter cell of a complete set of chromosomes. By inactivating both maternal and paternal copies of the gene encoding TTC5, Lin et al. demonstrate that tubulin autoregulation is essential for maintaining faithful chromosome segregation, with a modest increase in chromosome segregation errors in the absence of TTC5. Errors in chromosome inheritance are key drivers of tumorigenesis, so maintenance of the genome is important (10). However, Lin et al. have determined that cells with inactivated tubulin autoregulation are viable and can continue to survive and duplicate. This is unexpected because disruption of autoregulation would be predicted to yield runaway tubulin synthesis, which in turn should have severely disrupted microtubule assembly dynamics and function. Perhaps additional factors are involved, the activities of which might compensate, to some extent, for the lack of TTC5 activity.


Autoregulation of tubulin RNA

Tubulin messenger RNA (mRNA) instability is mediated by cotranslational binding of tetratricopeptide repeat protein 5 (TTC5) to the tubulin amino-terminal tetrapeptide MREI and activation of one or more ribosome-associated nucleases.




The new work also casts doubt on the notion that the tubulin concentration directly participates in the ribosomal complex with TTC5 and the MREI peptide. Instead, the findings of Lin et al. support a counterintuitive model in which cells ordinarily contain a cytosolic factor that prevents TTC5 binding to MREI-ribosome complexes and that this inhibitor is inactivated when the tubulin dimer concentration increases (see the figure).

The study of Lin et al. is a major step in deciphering a regulatory pathway for controlling expression of an important cellular product (tubulin) through cotranslationally mediated mRNA instability. It should be noted, however, that important steps in the autoregulatory pathway remain unidentified, including (i) the factor that inhibits TTC5 binding activity when tubulin levels are normal, (ii) the newly proposed autoregulatory signal generated by an increased pool of tubulin subunits, (iii) the nuclease(s) that mediate tubulin mRNA degradation, and (iv) the activation of those nucleases once TTC5 recognizes the nascent tubulin peptide.

This may only be the tip of the iceberg for cotranslational control of gene expression. Work in yeast suggests that mRNA decay through cotranslational regulation is widespread (11) and that it involves a 59-to-39 RNA exoribonuclease 1 (xrn1) (12). The TTC5 structure bound to a ribosome, as beautifully determined by Lin et al., provides clues that might allow the identification of additional regulators (perhaps containing the same tetratricopeptide repeats found in TTC5) that can bind the ribosome exit tunnel and simultaneously recognize targets encoded by different mRNAs.




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