Science - USA (2020-01-03)

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GRAPHIC: V. ALTOUNIAN/

SCIENCE

By Ofer Shoshani and Don W. Cleveland

O

ne of the first discoveries of gene

expression mediated by controlling

messenger RNA (mRNA) stability is

autoregulation of tubulin synthesis.

In this regulatory process, the con-

centration of tubulin subunits modu-

lates the stability of the mRNAs from which

they are translated ( 1 , 2 ). 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 tu-

bulin polypeptide to emerge from the ribo-

some ( 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 trans-

lating tubulin mRNA ( 4 , 6 – 8 ). More than 30

years later, on page 100 of this issue, Lin et

al. ( 9 ) identify tetratricopeptide repeat pro-

tein 5 (TTC5) as the regulator that binds to

nascent tubulin polypeptides.

With the exception of a high-resolution

confirmation that an elevated pool of tu-

bulin subunits selectively represses tubulin

synthesis ( 8 ), no progress toward under-

standing the autoregulation of tubulin ex-

pression 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 syn-

thesis was that it was the tubulin a/b 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 b-tubulin to identify

TTC5 as the protein that recognizes the na-

scent b-tubulin MREI tetrapeptide in com-

plex with the large ribosomal subunit.

a- and b-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 mo-

tors. During cell duplication, microtubule-

directed trafficking is essential for delivery to

each daughter cell of a complete set of chro-

mosomes. By inactivating both maternal and

paternal copies of the gene encoding TTC5,

Lin et al. demonstrate that tubulin autoregu-

lation is essential for maintaining faithful

chromosome segregation, with a modest in-

crease 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 autoregu-

lation are viable and can continue to survive

and duplicate. This is unexpected because

disruption of autoregulation would be pre-

dicted to yield runaway tubulin synthesis,

which in turn should have severely disrupted

microtubule assembly dynamics and func-

tion. Perhaps additional factors are involved,

the activities of which might compensate, to

some extent, for the lack of TTC5 activity.

The new work also casts doubt on the no-

tion 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 counterintui-

tive 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 di-

mer concentration increases (see the figure).

The study of Lin et al. is a major step in

deciphering a regulatory pathway for con-

trolling 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 nor-

mal, (ii) the newly proposed autoregulatory

signal generated by an increased pool of tu-

bulin subunits, (iii) the nuclease(s) that medi-

ate tubulin mRNA degradation, and (iv) the

activation of those nucleases once TTC5 rec-

ognizes 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 wide-

spread ( 11 ) and that it involves a 5 9 -to-3 9

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 tet-

ratricopeptide repeats found in TTC5) that

can bind the ribosome exit tunnel and simul-

taneously recognize targets encoded by dif-

ferent mRNAs. j

REFERENCES AND NOTES

1. T. J. Yen, D. A. Gay, J. S. Pachter, D. W. Cleveland, Mol. Cell.
   Biol. 8 , 1224 (1988).


2. D. W. Cleveland, M. A. Lopata, P. Sherline, M. W.
   Kirschner, Cell 25 , 537 (1981).


3. D. W. Cleveland, M. F. Pittenger, J. R. Feramisco, Nature
   305 , 738 (1983).


4. D. A. Gay, T. J. Yen, J. T. Lau, D. W. Cleveland, Cell 50 , 671
   (1987).


5. J. S. Pachter, T. J. Yen, D. W. Cleveland, Cell 51 , 283 (1987).


6. T. J. Yen, P. S. Machlin, D. W. Cleveland, Nature 334 , 580
   (1988).


7. A. Yonath, K. R. Leonard, H. G. Wittmann, Science 236 ,
   813 (1987).


8. I. Gasic, S. A. Boswell, T. J. Mitchison, PLOS Biol. 17 ,
   e3000225 (2019).


9. Z. Lin et al., Science 367 , 100 (2020).


10. U. Ben-David, A. Amon, Nat. Rev. Genet. 21 , 44 (2020).


11. V. Pelechano, W. Wei, L. M. Steinmetz, Cell 161 , 1400
    (2015).


12. W. Hu, T. J. Sweet, S. Chamnongpol, K. E. Baker, J. Coller,
    Nature 461 , 225 (2009).
    10.1126/science.aba0713

CELL BIOLOGY

Gene expression regulated by RNA stability

The factor responsible for autoregulation of tubulin RNA stability is identified

a/b-tubulin

concentration

Unknown

TTC5

inhibitor

Unknown

TTC5 inhibitor

is inactivated

Nuclease

II EERR TTC5 T

inhibiinh tor

TTC5

inhibiinh t

TTCTTCTTCTTCTTCTTTCT 5555

Transfer

RNA

TrTrTrTraaaaaa

RNARNARNARNARNARNARNA

5' Tubulin mRNA

5'

AAA

Unk

TTC

is ina

Un

Tubulin autoregulation activated

Tubulin

mRNA degraded

Ribosome

ncent

NORMAL

HIGH

Ludwig Institute for Cancer Research and Department of

Cellular and Molecular Medicine, University of California, San

Diego, La Jolla, CA 92093, USA. Email: [email protected]

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.

3 JANUARY 2020 • VOL 367 ISSUE 6473 29

Published by AAAS