STEFAN JENTSCH

  PREVIOUS PROJECTS
 

   UBIQUITIN-LIKE PROTEINS
 

Ubiquitin is one of the most highly conserved eukaryotic proteins known to date. Human ubiquitin differs from its yeast homolog by only three amino acid residues (out of 76 residues) and the proteins are functionally equivalent. It has now emerged that probably all eukaryotes also express proteins, which are related in sequence to ubiquitin, or function in an analogous manner. These proteins are involved in a variety of fundamental cellular processes, including protein sorting, protein degradation, DNA repair, cell division, autophagy, and apoptosis. Ubiquitin-like proteins fall into two clearly separate classes, making a distinction, also with respect to nomenclature, necessary (see Figure). Proteins of the first class function as modifiers analogously to ubiquitin. They exist either in a free form or, catalyzed by specific enzymes, become covalently attached to other proteins via their C-termini. We propose to term these proteins collectively "ubiquitin-like modifiers", or UBLs. Examples are SUMO-1 from higher eukaryotes and Smt3, its yeast ortholog. Proteins of the second class bear protein domains, which are related to ubiquitin, but otherwise these proteins are unrelated in sequence to each other. In contrast to UBLs, these proteins are not conjugated to other proteins. We call proteins of this class "ubiquitin-domain proteins", or UDPs. One example is the mammalian ubiquitin-related protein BAG-1 for which we showed (in collaboration with J. Hoehfeld) that it functions as a GrpE-like regulatory cofactor of the ATPase activity of the Hsc70 chaperone (Hoehfeld and Jentsch, 1997). 


Novel ubiquitin-like modifier: Rub1
We have identified two novel ubiquitin-like protein from yeast, which we termed Rub1 ("Related to UBiquitin") and Hub1 ("Homologous to UBiquitin"). Rub1 and Hub1 have close homologs in other species including humans. Whereas Hub1 is a UDP (see main homepage), Rub1 is a UBL and is affiliated with the ubiquitin system. Rub1 displays 53% amino acid sequence identity to ubiquitin  (see Figure).

Residues conserved between ubiquitin and Rub1 drawn into the known structure of ubiquitin (Vijay-Kumar et al., 1997, J. Mol. Biol. 194, 531-544.). Identical residues are shown in white, similar residues in light green, and dissimilar residues in dark green. Left and right views are shown and the C-terminus through which the protein is attached to other proteins is indicated. See text and Liakopoulos et al, (1998) for details.

We found that Rub1 conjugation to other cellular proteins requires at least three proteins in vivo. Ula1 and Uba3 are related to the amino- and carboxyl-terminal domains of the E1 ubiquitin-activating enzyme, respectively, and together fulfil E1-like functions for Rub1 activation. Rub1 conjugation (rubylation) also requires Ubc12, a protein related to E2 ubiquitin-conjugating enzymes, which functions analogously to E2 enzymes in Rub1-protein conjugate formation. Conjugation of Rub1 is not essential for normal cell growth and appears to be selective for a small set of substrates. Remarkably, we found that Cdc53/cullin, a common subunit of the multifunctional SCF ubiquitin ligase, is a major substrate for Rub1 conjugation. This indicates that the Rub1-conjugation pathway is functionally tied to the ubiquitin/proteasome system and may play a regulatory role.


 

Rub1/NEDD8 conjugation to cullin-2 is linked to von Hippel-Lindau (VHL) tumor suppressor function

(In collaboration with Tanja Büsgen, Alexander Brychzy, and Arnim Pause)

Yeast Rub1 has homologs in higher eukaryotic cells.  We showed that  the human protein NEDD8 is the functional ortholog of yeast Rub1  and that hCUL-1 and hCUL-2 (homologs of yeast Cdc53, see above) are modified by this protein. Whereas hCUL-1 is part of a human SCF complex, hCUL-2 assembles with elongin B/C  and the von Hippel-Lindau tumor suppressor protein pVHL , forming a protein complex, CBCVHL, that resembles SCF ubiquitin ligases.  We demonstrated that NEDD8-hCUL-2 conjugates are part of CBCVHL complexes in vivo. Remarkably, the formation of these conjugates is stimulated by the pVHL tumor suppressor. A tumorigenic pVHL variant, however, is essentially deficient in this activity. Thus, ligation of NEDD8 to hCUL-2 is linked to pVHL activity and may be important for pVHL tumor suppressor function. Moreover, our data indicate that modification of cullins by NEDD8 requires the existence of a pre-assembled ubiquitin-ligase complex.

Cullins are substrates of Rub1/NEDD8 (shown in blue) modification. The yeast cullin Cdc53 and human hCUL-1 are part of SCF ubiquitin ligases, which also contain Skp1 and Roc1(Rbx1). hCUL-2, however, binds to elongin B, elongin C, and ROC1, forming a CBC ubiquitin ligase. SCF ubiquitin ligases associate with substrate (S) -specifying F-box proteins. CBC associates with the tumor suppressor protein pVHL. Modification of cullins by Rub1/NEDD8 depends on pre-assembled ubiquitin ligases.

See Liakopoulos et al.(1999) for details.

Conjugation of the ubiquitin-like protein NEDD8 to cullin-2 is linked to von Hippel-Lindau (VHL) tumor suppressor function.
Liakopoulos, D., Buesgen, T., Brychzy, A.,  Jentsch, S., and Pause, A. (1999).
Proc. Natl. Acad. Sci. USA 96, 5510-5515.

A novel protein modification pathway related to the ubiquitin system.
Liakopoulos, D., Doenges, G., Matuschewski, K., and Jentsch, S. (1998).
EMBO J.17, 2208-2214.

The ubiquitin-like proteins SMT3 and SUMO-1 are conjugated by the UBC9 E2 enzyme.
Schwarz, S. E., Matuschewski, K., Liakopoulos, D., Scheffner, M., and Jentsch, S. (1998).
Proc. Natl. Acad. Sci. USA 95, 560-564.

GrpE-like regulation of the Hsc70 chaperone by the anti-apoptotic protein BAG-1.
Hoehfeld, J. and Jentsch, S. (1997).
EMBO J. 16, 6209-6216.

mUBC9, a novel adenovirus E1A-interacting protein that complements a yeast cell cycle defect.
Hateboer, G., Hijmans, E. M., Nooij, J.B.D., Schlenker, S., Jentsch, S., and Bernards, R. (1996).
J. Biol. Chem.271, 25906-25911.

Reviews:

SUMO, ubiquitin's mysterious cousin
Müller S., Hoege C., Pyrowolakis G. and Jentsch S. (2001).
Nature Reviews Mol. Cell Biol.2, 202-213.

Ubiquitin and its kin: how close are the family ties?
Jentsch, S. and Pyrowolakis, G. (2000).
Trends Cell Biol.10, 335-342.

Ubiquitous déjà vu.
Jentsch, S. and Ulrich, H.D. (1998).
Nature 393, 321-323.

 


 
 

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