Botanical Studies (2006) 47: 389-395.
Corresponding author: E-mail: email@example.com.
tw; Fax: 886-2-26515600.
Heat-shock proteins (Hsps) are transiently induced
by elevated temperature to confer protection against
the detrimental effect of heat stress, a phenomenon
known as heat shock (HS) response (Lindquist, 1986).
Conserved Hsps, such as Hsp100, Hsp90, Hsp70, Hsp60,
and the small Hsps, have been well characterized as
molecular chaperones, responsible for the acquisition of
thermotolerance by maintaining the homeostasis of protein
folding in cells (Vierling, 1991; Parsell and Lindquist,
1993). With the progress of genomic and functional
genomic research, novel Hsps have been identified from
various species (Gasch et al., 2000; Helmann et al., 2001;
Shockley et al., 2003; Pysz et al., 2004; Rizhsky et al.,
2004; Busch et al., 2005). However, the functions of many
of these novel Hsps are unknown, thus hindering a further
understanding of this important physiological process.
In an attempt to discover the unique features in the
plant HS response, we have previously identified and
characterized a novel Hsp gene, Hsa32, which is highly
conserved in land plants but not present in most other
organisms (Liu et al., 2006). Suppression of Hsa32
expression by T-DNA insertion or RNAi transgene led to
significant defects in acquired thermotolerance during a
long recovery period after acclimation, which suggests an
important role for the protein in a protection mechanism
needed in plants (Charng et al., 2006). How Hsa32 exerts
its function is currently unknown. The gene encodes
an HS-induced 32-kD protein dissimilar to any well-
characterized Hsps but weakly similar (about 35%) to
the bacterial (2R)-phospho-3-sulfolactate (PSL) synthase
(Graham et al., 2002). The structural homology to PSL
synthase provides a possible clue.
In archaeal methanogens, PSL synthase or ComA cata-
lyzes the first step in the biosynthetic pathway of coen-
zyme M (CoM), which serves as a cofactor for methane
production (Graham et al., 2002). In this reaction, PSL is
formed by the stereospecific addition of sulfite to phos-
phoenolpyruvate. Since plants do not have the other CoM
biosynthesis genes, it was suggested that the plant PSL
synthase-related protein is involved in the biosynthesis of
sulfoquinovosyl diacylglycerol (SQDG) (Graham et al.,
hsa32, a phosphosulfolactate synthase-related heat-
shock protein, is not involved in sulfolipid biosynthesis
Nai-Yu LIU, Wan-Jen HSIEH, Hsiang-Chin LIU, and Yee-Yung CHARNG*
Agricultural Biotechnology Research Center, Academia Sinica, Taipei, TAIWAN 11529, Republic of China
(Received November 1, 2005; Accepted March 15, 2006)
. Hsa32 is a novel heat-shock protein (Hsp) mainly found in land plants. Recently, it was
shown to be essential for acquired thermotolerance following a long recovery after acclimation heat-shock
(HS) treatment. Without Hsa32, Arabidopsis mutant plants become more sensitive to severe HS than wild-
type plants due to faster decay of a previously acquired protection. Sequence homology showed Hsa32 to be
a phosphosulfolactate synthase-related protein, and it was proposed to be involved in the biosynthesis of sul-
foquinovosyl diacylglycerol (SQDG), one of the major sulfur-containing glycolipids in the chloroplast thyla-
koid membrane. Currently, Sqd1 and Sqd2 are known to catalyze the consecutive reactions in the biosynthetic
pathway of the sulfolipid. In this study, we examine Hsa32¡¦s possible involvement in an alternative pathway
that bypasses Sqd1. Our analysis of the wild type and Hsa32 T-DNA knockout mutant plants revealed no
significant differences in SQDG accumulation. In addition, the Arabidopsis mutant with a disrupted Sqd1
gene did not synthesize SQDG, which discounts the existence of an alternative pathway. The Sqd1 and Sqd2
knockout mutants, both lacking SQDG, did not show the same defect in acquired thermotolerance as did the
Hsa32 null mutant, which suggests that the sulfolipid level is not related to the HS-sensitive phenotype. Our
data suggest that Hsa32 is not involved in SQDG biosynthesis.
Keywords: Arabidopsis; Sulfolipid; Thermotolerance.
Abbreviations: HS, Heat-shock; UDP-SQ, uridine diphosphate-sulfoquinovose; SQDG, sulfoquinovosyl dia-