August 1, 2005: Identification of a novel
ubiquitin-activating enzyme E1 variant in the testis supports the importance of
regulated proteolysis in human/mammalian spermatogenesis.
Commentary
by Peter Sutovsky for Acta Biochimica et Biophysica
Sinica
Last year‘s Nobel Prize in chemistry was awarded to Aaron Ciechanover, Avram
Hershko and Irwin Rose for their discovery of the ATP-dependent, proteolytic
ubiquitin system. Due to its unique combination of substrate specificity and
high evolutionary conservation, the ubiquitin system is likely to play a role
in any body function. Besides general housekeeping and protein recycling, some
of the best studied examples of the function of ubiquitin system are those of
cell cycle regulation, the endoplasmatic reticulum-associated protein quality
control and antigen presentation within immune system (reviewed in [1]). No less important is the
functioning of ubiquitin system in varied pathologies including but not limited
to Alzheimer’s disease, retroviral infection (e.g. HIV) and liver cirrhosis
brought about by alcohol abuse. The central dogma of the ubiquitin system is
that the small chaperone protein ubiquitin, covalently linked to a substrate
protein in a tandem fashion, marks the said substrate for proteolysis by the 26
S proteasome, a multi-subunit protease complex typically composed of a 19 S
regulatory subunit and a 20 S core [2].
This stable but reversible postranslational modification is aided by a specific
set of ubiquitin activating enzymes (E1), ubiquitin-conjugating enzymes (E2)
and substrate specific ubiquitin ligases (E3).
Despite the current boom of ubiquitin
research, the field of reproductive biology largely stood away from studying
the ubiquitin system, with few notable exceptions including the implication of
the ubiquitin system in the surveillance mechanisms for epididymal sperm
maturation and organelle inheritance after fertilization (reviewed in [3]), the function of sperm
acrosome-borne proteasome during fertilization [4-6], and the knock out studies of ubiquitin conjugating enzymes in
the testis [7-9]. The later studies
demonstrated that genetic ablation of ubiquitin conjugating enzymes in the
testis can halt spermatogenesis at well defined stages, including spermatocyte
meiosis [8] and spermatid elongation
[9]. Several papers support this
notion, showing the participation of ubiquitin system in the degradation of
spermatid cytosolic proteins and organelles, and the biogenesis of sperm
accessory structures [10,11].
While many enzymes of the ubiquitin system
and ubiquitin itself are phylogenetically conserved, it is likely that
testis-specific gene products related to the ubiquitin system and/or those
derived by alternative splicing of transcripts present also in somatic cells
participate in the process of spermatogenesis. Along this line, the paper by
Zhu et al. [12], published last year in the Acta Biochmica et Biophysica Sinica, reports the identification of a new
ubiquitin activating enzyme of E1 type, designated nUBE1L, that is
predominantly expressed in the testis. Although no functional studies were
performed, it appears that the enzymes is highly expressed in the adult testis,
and may thus be involved in spermatogenesis. The aminoacid sequence of the
nUBE1L enzyme displays typical features of an ubiquitin activating enzyme,
including the Thif-domain, two UBACT domains and a conserved cysteine residue
upstream of the UBACT site, implicated in the formation of thiol-ester bond
with a monoubiquitin molecule during ubiquitin activation. Further, according
to their observations, the authors propose that a common theme of alternative
splicing of somatic-cell-like transcripts of ubiquitin-conjugating enzymes in
the testis provides a mechanism guiding spermatogenesis. This is a reasonable
proposition, as we see other genes not related to ubiquitin system (reviewed in
[13]) being transcribed and
translated in that fashion in the testis. Although the present study does not
provide clues as to when and in which cell type within the testis this novel E1
enzyme functions, the report supports the view that developmentally regulated,
ubiquitin-dependent proteolysis helps controlling spermatogenesis.
Alternatively, it cannot be ruled out that this and/or other enzymes become
sequestered in the mature spermatozoa and may be involved in epididymal sperm
maturation or in the sperm function during fertilization. While similar
evidence is only now being generated in mammals, studies in ascidians have
shown that the complement of sperm borne ubiquitinating enzymes participated in
the ubiquitination and degradation of the sperm receptor on the egg vitelline
envelope [4].
Peter Sutovsky, PhD
Assistant Professor
University of Missouri-Columbia
S141 ASRC
920 East Campus Drive
Columbia, MO 65211-5300
Telephone: (573) 882-3329
Fax: (573) 884-5540
http://www.missouri.edu/~reprphys/sutovskyp.htm
REFERENCES
1
Glickman, M.H., and Ciechanover, A. The ubiquitin-proteasome
proteolytic pathway: destruction for the sake of construction. Physiol.
Rev. 2002; 82, 373-428.
2
Rechsteiner, M. The 26 S proteasome. In: Ubiquitin and the Biology of
the Cell. Peters J-M, Harris JR, Finley D, editors. New York: Plenum Press
1998; pp 147-189.
3
Baska KM Sutovsky P. Protein modification by ubiquitination and is
consequences for spermatogenesis, sperm maturation, fertilization and
pre-implantation embryonic development. In: New impact on protein modifications
in the regulation of reproductive system. Tokumoto, T, Ed., Research Signpost,
Kerala 2005; 83-114.
4
Sawada H, Sakai N, Abe Y, Tanaka E, Takahashi Y, Fujino J, Kodama E,
Takizawa S, Yokosawa H. Extracellular ubiquitination and proteasome-mediated
degradation of the ascidian sperm receptor. Proc Natl Acad Sci U S A. 2002;
99:1223-1228.
5
Morales P, Kong M, Pizarro E, Pasten C. 2003. Participation of the
sperm proteasome in human fertilization. Hum Reprod. 2003; 18(5):1010-7.
6
Sutovsky, P., Manandhar G., McCauley, T.C., Caamaño JN, Sutovsky, M.,
Thompson WE, and Day, B.N. Proteasomal interference prevents zona pellucida
penetration and fertilization in mammals. Biol Reprod. 2004; 71:1625-1637.
7
Baarends WM, Roest HP, Grootegoed JA. The ubiquitin system in
gametogenesis. Mol Cell Endocrinol 1999; 151:5-16.
8
Baarends WM, Wassenaar E, Hoogerbrugge JW, van Cappellen G, Roest HP,
Vreeburg J, Ooms M, Hoeijmakers JH, Grootegoed JA. Loss of HR6B
ubiquitin-conjugating activity results in damaged synaptonemal complex
structure and increased crossing-over frequency during the male meiotic
prophase. Mol Cell Biol. 2003; 23(4):1151-62.
9
Roest HP, van Klaveren J, de Wit J, van Gurp CG, Koken MH, Vermey M,
van Roijen JH, Hoogerbrugge JW, Vreeburg JT, Baarends WM, Bootsma D, Grootegoed
JA, Hoeijmakers JH. Inactivation of the HR6B ubiquitin-conjugating DNA repair
enzyme in mice causes male sterility associated with chromatin modification.
Cell. 1996; 86(5):799-810.
10
Escalier D. New insights into the assembly of the periaxonemal
structures in mammalian spermatozoa. Biol Reprod. 2003; 69(2):373-8. Epub 2003
Apr 2.
11
Haraguchi CM, Mabuchi T, Hirata S, Shoda T, Hoshi K, Akasaki K, Yokota
S. Chromatoid bodies: aggresome-like characteristics and degradation sites for
organelles of spermiogenic cells. J Histochem Cytochem. 2005; 53(4):455-65.
12
Zhu H, Zhou ZM, Huo R, Huang XY, Lu L, Lin M, Wang LR, Zhou YD, Li JM,
Sha JH. Identification and characteristics of a novel E1 like gene nUBE1L in
human testis. Acta Biochim Biophys Sin. 2004; 36(3):227-34.
13
Venables JP. Alternative splicing in the testes. Curr Opin Genet Dev.
2002; 12(5):615-19.