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Molecular and Cellular Biology, September 2000, p. 6568-6578, Vol. 20, No. 17
Department of
Medicine1 and Department of Anatomy and
Cell Biology,2 McGill University, Montreal,
Canada
Received 22 March 2000/Returned for modification 20 April
2000/Accepted 29 May 2000
Ubiquitin-specific processing proteases (UBPs) presently form the
largest enzyme family in the ubiquitin system, characterized by a core
region containing conserved motifs surrounded by divergent sequences,
most commonly at the N-terminal end. The functions of these divergent
sequences remain unclear. We identified two isoforms of a novel
testis-specific UBP, UBP-t1 and UBP-t2, which contain identical core
regions but distinct N termini, thereby permitting dissection of the
functions of these two regions. Both isoforms were germ cell specific
and developmentally regulated. Immunocytochemistry revealed that UBP-t1
was induced in step 16 to 19 spermatids while UBP-t2 was expressed in
step 18 to 19 spermatids. Immunoelectron microscopy showed that UBP-t1
was found in the nucleus while UBP-t2 was extranuclear and was found in
residual bodies. For the first time, we show that the differential
subcellular localization was due to the distinct N-terminal sequences.
When transfected into COS-7 cells, the core region was expressed
throughout the cell but the UBP-t1 and UBP-t2 isoforms were
concentrated in the nucleus and the perinuclear region, respectively.
Fusions of each N-terminal end with green fluorescent protein yielded the same subcellular localization as the native proteins, indicating that the N-terminal ends were sufficient for determining differential localization. Interestingly, UBP-t2 colocalized with anti-
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Divergent N-Terminal Sequences Target an Inducible
Testis Deubiquitinating Enzyme to Distinct Subcellular
Structures


-tubulin immunoreactivity, indicating that like several other components of the
ubiquitin system, a deubiquitinating enzyme is associated with the
centrosome. Regulated expression and alternative N termini can
confer specificity of UBP function by restricting its temporal and
spatial loci of action.
*
Corresponding author. Mailing address: Polypeptide
Laboratory, McGill University, Strathcona Anatomy & Dentistry Building, Room W-315, 3640 University St., Montreal, Quebec, Canada H3A 2B2.
Phone: (514) 398-4101. Fax: (514) 398-3923. E-mail:
cxwg{at}musica.mcgill.ca.
Present address: Department of Biology, Université de
Strasbourg, Strasbourg, France.
Present address: Centre d'Immunologie de Marseille Luminy,
Université de Marseilles, Marseilles, France.
§
Present address: Institut National de la Recherche Agronomique,
Centre de Theix, Ceyrat, France.
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