TIBS 22 - OCTOBER 1997 EDITORIAL

Extinction is forever

Protein degradation is a most-ancient concern of biochemists. Proteases were among the first enzymes to be purified and charactedsed, both structural]y and mechanistically, as David Blow reminds us later in this special issue of TiBS. The leading lights of 50 years ago struggled to understand why some proteins were nutritionally deficient, and made eXhaus- tive quantitative measurement of ex- creted nitrogen that discouraged many more budding biochemists than it en- lightened; how many readers recall the Conway dish, the Van Slyke apparatus? The idea that proteolysis can be regu- lated as precisely as protein synthesis has taken a long time to become dear and credible, although one shouM not forget the extraordinary successes in working out the delicately poised pro- tease cascades involved in blood clotting and the complement system, and how snake venoms work. These are sensitively regulated, highly amplified, irreversible yet self limiting processes - but they occur outside cells.

Within the cell, things are different. Leaving aside for the moment the mun- dane business of cleansing the cell of damaged proteins (although as far as 1 know, how damaged proteins are identi- fied is but dimly understood), lysosomes and cathepsins play an important role in the immune system, as described by Beatrice Fineschi and Jim Miller in their article on antigen processing. In the cyto- plasm, however, ubiquitin seems to be at the heart of regulated intracellular turnover, and we are grateful to Alex Varshavsky for his account of his adven- tures with that system. Even in this well- worked area, however, there are fresh surprises, and Mary Dasso writes about the recently recognised ubiquitin-related protein, SUMO-1, which seems to have an important role in traffic across the nu- clear membrane, rather than in acting as a tag that marks proteins for destruction.

Ubiqaitin itself does more than sim- ply scavenge, or set intrinsic lifespans. David Lane and Peter Hall describe the recent findings from their own laboroto- ries and those of Moshe Oren and Karcn Vousden that Mdm2 not only binds to p53, but also targets it for ubiquitination and destruction. Given the central role of p53 in regulating cell death, it is per- haps surprising that the death process

itself, while involving cascades of dedi- cated proteases, does not yet (at least) seem to require the crucial services of the ubiquitin system as Bill Earnshaw and his colleagues Pascal Villa and Scott Kaufmann relate on p. 388 of this issue.

Using proteo]ysis to regulate cellular processes seems like a wonderful trick to use where irreversible committment is required, like apoptosis or, what spe- cially interests me, cell-cycle transitions - and hence the title of this piece, which I found as a surprising advertising button outside a Heathrow branch of the Body Shop. Certain proteins stand in the way of progress, so to speak, and must be se- lectively culled in the natural course of bio- logical events, just as superfluous cells have to be sacrificed by apoptosis dur- ing the construction of a paw or a brain. Cells actually change their protein com- position as they undergo cell-cycle tran- sitions, or change from one cell type into another. You cannot traverse a whole cell cycle, I believe, by cleverly networked protein kinases and phosphatases alone!

Obviously, the other face of the proteolytic coin is the need for new tran- scription and translation, but getting rid of the old unwanted if not downright ob- structive proteins turns out to be an es- sential part of cellular life. ! first appreci- ated this point most clearly by witnessing the astonishing behaviour of the mitotic cyclins, which disappear abruptly just before sister chromatid separation oc- curs in fertilised clam and sea urchin eggs as they entered anaphase. I would like to briefly outline what we do and don't know about this process.

The proteolysis of the mitotic cyclins turns out to be amazingly selective and strikingly richly regulated; it is probably spacially restricted as well. It depends on a 9-residue motif in the unstructured N-terminus of the mitotic cyclins. The sequence of the 'destruction box' is not specially highly conserved except for the single arginine residue that marks its start. From early on, it looked probable that there might be othel" targets for this extraordinary proteolytic system, and evidence for this idea has recently come from studies of frog egg cell-free sys- terns, and of budding and fission yeast. Thus, a small protein called Pdsl in bud- ding yeast and Cut2 in fission yeast are other essential targets for destruction

bo×°me,~ iated proteo~ysis. They appear to be key regulators of the metaphase to anaphase transition, a~though their mo]- ecular modes of action are as yet un- clear, and one thing they are certaiNy not is the long-sought sister chromatid glue preteins. Mutations in their de- struction boxes stabilise cyclins, Pdsl and Cut2 alike with disastrous conse- quences for cell-cycle progression.

Perhaps surprisingly, though, it is not known what recognises the destruction box, which looks for all the world like a typical recognition tag for some other protein. Nor is it clear why some boxes can be moved around in modular fashion, conferring post-mitotic instability on their recipients, while others (like the destruc- tion box of cyclin A) seem only to work in their original context. What is pretty clear, however, is that a complicated pro- tein machine called the APC (anaphase- promoting complex) or cyclosome is necessary for destroying destruction-box proteins, and that at least in cell-free systems, the APC strongly promotes ubiquitination of its targets. Other el- ements are needed as well, however, of which the yeast gene Csel is perhaps the most obscure- not a part of the APC, or a ubiquitin ligase, or a component of the proteasome, but seemingly a nuclear shuttling protein. Fizzy and fizzy-related (first recognised in Drosophila, but with yeast and vertebrate bomologues) also play a key role, and seem to link the spindle checkpoint pathway with the de- struction machinery. We hope to carry a full article on this fascinating system in the near future.

The final fate of ubiquitinated pro- teins by degradation in the proteasome is discussed on p. 399 by Wolfgang Baumeister and his colleagues.

Much effort by drug companies has developed some very nice, specific pro- tease inhibitors that can block viral replication, assembly or infection. Wally Mangel and his group describe the fasci- nating properties and structure of the adenovirus protease, which is an essen- tial element in the viral life cycle, and seems to require DNA for its activity. Will wonders never cease?

TIM HUNT

ICRF Clare Hall Laboratories, South Mimms, Herts, UK EN6 3LD.

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