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J Biol Chem, Vol. 273, Issue 29, 18435-18442, July 17, 1998
From the The ubiquitin pathway is a major system for
selective proteolysis in eukaryotes. However, the mechanisms underlying
substrate selectivity by the ubiquitin system remain unclear. We
previously identified isoforms of a rat ubiquitin-conjugating enzyme
(E2) homologous to the Saccharomyces cerevisiae class I E2
genes, UBC4/UBC5. Two isoforms, although 93%
identical, show distinct features. UBC4-1 is expressed ubiquitously,
whereas UBC4-testis is expressed in spermatids. Interestingly, although
these isoforms interacted similarly with some ubiquitin-protein ligases
(E3s) such as E6-AP and rat p100 and an E3 that conjugates ubiquitin to
histone H2A, they also supported conjugation of ubiquitin to distinct
subsets of testis proteins. UBC4-1 showed an 11-fold greater ability to support conjugation of ubiquitin to endogenous substrates present in a
testis nuclear fraction. Site-directed mutagenesis of the UBC4-testis
isoform was undertaken to identify regions of the molecule responsible
for the observed difference in substrate specificity. Four residues
(Gln-15, Ala-49, Ser-107, and Gln-125) scattered on surfaces away from
the active site appeared necessary and sufficient for UBC4-1-like
conjugation. These four residues identify a large surface of the E2
core domain that may represent an area of binding to E3s or substrates.
These findings demonstrate that a limited number of amino acid
substitutions in E2s can dictate conjugation of ubiquitin to different
proteins and indicate a mechanism by which small E2 molecules can
encode a wide range of substrate specificities.
Identification of Amino Acid Residues in a Class I
Ubiquitin-conjugating Enzyme Involved in Determining Specificity of
Conjugation of Ubiquitin to Proteins
,,
Department of Medicine, Polypeptide
Laboratory, McGill University, Montreal, Quebec H3A 2B2, Canada and the
¶ Department of Biochemistry and the Montreal Joint Centre for
Structural Biology, McGill University, Montreal,
Quebec H3G 1Y6, Canada
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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