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Molecular and Cellular Biology, November 2001, p. 7673-7681, Vol. 21, No. 22
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.22.7673-7681.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Dual Function for U2AF35 in
AG-Dependent Pre-mRNA Splicing
Sabine
Guth,1
Thomas Ø.
Tange,1,
Esther
Kellenberger,2 and
Juan
Valcárcel1,*
Gene Expression
Programme1 and Structural Biology
Programme,2 European Molecular Biology
Laboratory, 69117 Heidelberg, Germany
Received 30 July 2001/Accepted 27 August 2001
The splicing factor U2AF is required for the recruitment of U2
small nuclear RNP to pre-mRNAs in higher eukaryotes. The 65-kDa subunit
of U2AF (U2AF65) binds to the polypyrimidine (Py) tract
preceding the 3' splice site, while the 35-kDa subunit
(U2AF35) contacts the conserved AG dinucleotide at the 3'
end of the intron. It has been shown that the interaction between
U2AF35 and the 3' splice site AG can stabilize
U2AF65 binding to weak Py tracts characteristic of
so-called AG-dependent pre-mRNAs. U2AF35 has also been
implicated in arginine-serine (RS) domain-mediated bridging
interactions with splicing factors of the SR protein family
bound to exonic splicing enhancers (ESE), and these interactions can
also stabilize U2AF65 binding. Complementation of the
splicing activity of nuclear extracts depleted of U2AF by
chromatography in oligo(dT)-cellulose requires, for some pre-mRNAs,
only the presence of U2AF65. In contrast, splicing of a
mouse immunoglobulin M (IgM) M1-M2 pre-mRNA requires both U2AF
subunits. In this report we have investigated the sequence elements
(e.g., Py tract strength, 3' splice site AG, ESE) responsible for the
U2AF35 dependence of IgM. The results indicate that (i) the
IgM substrate is an AG-dependent pre-mRNA, (ii) U2AF35
dependence correlates with AG dependence, and (iii) the identity of the
first nucleotide of exon 2 is important for U2AF35
function. In contrast, RS domain-mediated interactions with SR proteins
bound to the ESE appear to be dispensable, because the purine-rich ESE
present in exon M2 is not essential for U2AF35 activity and
because a truncation mutant of U2AF35 consisting only of
the pseudo-RNA recognition motif domain and lacking the RS domain is
active in our complementation assays. While some of the effects of
U2AF35 can be explained in terms of enhanced
U2AF65 binding, other activities of U2AF35 do
not correlate with increased cross-linking of U2AF65 to the
Py tract. Collectively, the results argue that interaction of
U2AF35 with a consensus 3' splice site triggers events in
spliceosome assembly in addition to stabilizing U2AF65
binding, thus revealing a dual function for U2AF35 in
pre-mRNA splicing.
*
Corresponding author. Mailing address: Gene Expression
Programme, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany. Phone: 49-6221-387 156. Fax: 49-6221-387 306. E-mail: juan.valcarcel{at}embl-heidelberg.de.

Present address: Department of Biochemistry, Brandeis University,
Waltham,
Mass.
Molecular and Cellular Biology, November 2001, p. 7673-7681, Vol. 21, No. 22
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.22.7673-7681.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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