Plant Molecular Biology 41: 65–73, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
The Arabidopsis ACTIN-RELATED PROTEIN 2 (AtARP2) promoter
directs expression in xylem precursor cells and pollen
Ulrich Klahre and Nam-Hai Chua
Laboratory of Plant Molecular Biology, Rockefeller University, 1230 York Avenue, New York, NY 10021-6399,
author for correspondence)
Received 29 September 1998; accepted in revised form 28 June 1999
Key words: actin-related protein, Arabidopsis, pollen, xylem
To investigate the role of the actin cytoskeleton in plant growth and development, we have cloned and determined
the DNA sequence of a gene encoding an actin-related protein (arp) from Arabidopsis thaliana (AtARP2)and
studied its expression patterns. A. thaliana appears to have only one AtARP2 gene which contains 14 introns,
an unusually large number when compared to 4 or 5 introns in the actin genes isolated so far. The predicted
protein shows high homology to and shares typical peptide insertions with arp2 proteins from other organisms.
The AtARP2 transcript is present in all plant tissues, at a very low level, and is down-regulated by light. Promoter-
GUS expression studies showed that the AtARP2 promoter directs activity predominantly in a very small number
of cells immediately adjacent to the xylem in all organs. In addition, strong expression was observed in pollen
grains. We discuss the potential role of an arp2/3 complex in plant development.
Actin is among the proteins that are most highly
conserved throughout evolution (Herman, 1993). The
typical tertiary structure of actin can also be found in
many related proteins including the actin-related pro-
teins (arps) and proteins resembling hsc70 and sugar
kinases (Frankel and Mooseker, 1996). The latter
showa lower amino acid homologywith actin, but still
retain the characteristic actin fold.
Arps share only 35–55% of amino acid sequence
homology with conventional actin, but they are highly
conserved throughout evolution themselves. Whereas
conventional actins are very abundant proteins, arps
are expressed at low levels. These proteins have been
classiﬁed based on their conserved amino acid se-
quences (Schroer et al., 1994) and each class is char-
acterized by small peptide insertions or deletions at
speciﬁc locations when compared with conventional
actin. Notwithstandingthe sequence homology to con-
The nucleotide sequence data reported will appear in the EMB,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession number AJ095912.
ventional actin, available data suggest that arps fulﬁll
highly diverse functions, a fact that is also manifested
by the wide spectrum of approaches that led to their
identiﬁcation (Frankel and Mooseker, 1996; Frankel
et al., 1997; Jiang and Stillman, 1996; Schwob and
Although the Saccharomyces cerevisiae genome
contains ten different types of arps (Poch and Winsor,
1997), so far three main classes of arps have been
identiﬁed. Members of each class have been isolated
from a variety of organisms. Arp1 is most similar to
conventionalactin (ca. 47–56%identical)and has pep-
tide insertions or deletionsnear threonine 229 of actin.
Arp1 was ﬁrst found in the dynactin complex and
has been shown to interact with microtubules (Schroer
et al., 1994; Allan, 1994) and actin (Schafer et al.,
1994). This arp is very low in abundance, but it can
form ﬁlaments, just like actin, although the ﬁlaments
are substantially shorter (Schafer et al., 1994; Frankel
and Mooseker, 1996).
Arp2 (ca. 47% identity with actin) has a major
peptide insertion near actin alanine 321 and arp3 pro-
teins are least similar to actin (ca. 35% identity) and