Miten ubikitiini syntyy?

https://www.nature.com/articles/srep12836
rticle | Open | Published: 03 August 2015

The de novo synthesis of ubiquitin: identification of deubiquitinases acting on ubiquitin precursors

• Cláudia P. Grou
• , Manuel P. Pinto
• , Andreia V. Mendes
• , Pedro Domingues
•  & Jorge E. Azevedo

Scientific Reports volume 5, Article number: 12836 (2015) | Download Citation

 Protein ubiquitination, a major post-translational modification in eukaryotes, requires an adequate pool of free ubiquitin. Cells maintain this pool by two pathways, both involving deubiquitinases (DUBs): 

(1)  recycling of ubiquitin from ubiquitin conjugates and 

(2)  processing of ubiquitin precursors synthesized de novo.

 Although many advances have been made in recent years regarding ubiquitin recycling, our knowledge on ubiquitin precursor processing is still limited, and questions such as when are these precursors processed and which DUBs are involved remain largely unanswered.

 Here we provide data suggesting that two of the four mammalian ubiquitin precursors, UBA52 and UBA80, are processed mostly post-translationally whereas the other two, UBB and UBC, probably undergo a combination of co- and post-translational processing.

Using an unbiased biochemical approach we found that UCHL3, USP9X, USP7, USP5 and Otulin/Gumby/FAM105b are by far the most active DUBs acting on these precursors. The identification of these DUBs together with their properties suggests that each ubiquitin precursor can be processed in at least two different manners, explaining the robustness of the ubiquitin de novo synthesis pathway.

 Introduction

 Protein ubiquitination is one of the most important post-translational modifications. It is used to control the half-life of proteins via the ubiquitin-proteasome system¹ and also to regulate in a reversible manner a myriad of biological pathways such as DNA damage repair², signal transduction³, and protein sorting^4,5. The fine-tuned regulation of biological processes by ubiquitination relies, on one hand, on the action of the ubiquitin-conjugating cascade, a complex set of enzymes that conjugate ubiquitin to proteins^6,7, and on the other hand, on deubiquitinases (DUBs), hydrolases that remove ubiquitin molecules from the modified proteins, thus ensuring the transient nature of ubiquitination.DUBs are a diverse class of proteases comprising about 100 members in mammals (reviewed in ref. 8).

In addition to removing ubiquitin from ubiquitinated proteins, DUBs are also involved in the de novo synthesis of ubiquitin. The participation of DUBs in this process is mandatory because ubiquitin is synthesized by cytosolic ribosomes in a precursor form that must be processed to yield active ubiquitin (reviewed in ref. 9).

 In mammals there are four such precursors, each encoded by a different gene^10,11.

•  UBA52 and 
•  UBA80

 (hereafter referred to as Ub-RPs) comprise a single ubiquitin molecule C-terminally fused to a ribosomal protein, L40 and S27A, respectively^12,13.

•  UBB  and 
• UBC

 (polyUbs) are polymers of ubiquitins (3–4 and 7–10 ubiquitins, respectively, depending on the organism) linked in a “head-to-tail” fashion, followed by a variable C-terminal extension that goes from a single amino acid to a few dozens¹⁴.