{PDOC50842}
{PS50842; EXPANSIN_EG45}
{PS50843; EXPANSIN_CBD}
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* Expansin family profiles *
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Expansins  are  secreted proteins of 25 to 27 Kd that were isolated first from
young  cucumber seedling and subsequently from other plant tissues. Expression
of  expansin  genes  correlates  with  growth  of  cells. Increase in expansin
content  also  occurs during fruit ripening. Expansins act on the cell wall to
promote  its  extensibility.  The model for its mechanism of action postulates
that  expansins  break  non-covalent  bonds between cell-wall polysaccharides,
thereby permitting pressure dependent expansion of the cell [1,2].

Group-I  pollen  allergens  of  grasses  have limited but significant sequence
homology to expansin. These proteins are the main causative agent of hay fever
and  seasonal  asthma  induced  by  grass  pollen. Extracts containing group-I
allergens  are  also  active in loosening cell-walls. Group-I pollen allergens
and   related   proteins   in  vegetative  tissues  have  been  classified  as
beta-expansins,  whereas  the earlier discovered expansins are now referred to
as alpha-expansins [3].

Expansin-like  proteins are also found in some fungi. In Trichoderma reesei an
expansin-like protein (Cel12A) acts as a glycoside hydrolase on xyloglucan and
1-4 beta-glucan. These hydrolytic actions differ from the action by expansins,
which induce wall extension by a non-hydrolytic mechanism [4].

Expansins consist  of  two  domains closely packed and aligned so as to form a
long, shallow  groove  with  potential  to bind a glycan backbone of ~10 sugar
residues (see  <PDB:2HCZ>).  The  N-terminal  cysteine-rich domain has distant
sequence similarity  to  family-45  endoglucanases  (EG45-like domain). It has
noteworthy, but  incomplete,  conservation of the catalytic site identified in
EG45 enzymes.  The ~120-residue EG45-like domain has three disulfide bonds and
the six participating cysteines are highly conserved. Its fold is dominated by
a six-stranded  beta-barrel flanked by short loops and alpha-helices. The ~90-
residue C-terminal domain may function as a cellulose-binding domain (CBD). It
is composed of eight beta-strands assembled into two antiparallel beta-sheets.
The two  beta-sheets  are  at  slight  angles  to  each other and form a beta-
sandwich similar to the Ig fold [5].

Two  profiles  were  developed  for  this  family,  the first one picks up the
EG45-like  domain  while  the  second  profile  detects  the cellulose-binding
domain.

-Sequences known to belong to this class detected by the first profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Sequences known to belong to this class detected by the second profile: ALL.
-Other sequence(s) detected in Swiss-Prot: NONE.

-Last update: January 2007 / Text revised.

[ 1] Cosgrove D.J.
     "Loosening of plant cell walls by expansins."
     Nature 407:321-326(2000).
     PubMed=11014181; DOI=10.1038/35030000
[ 2] Lee Y., Choi D., Kende H.
     "Expansins: ever-expanding numbers and functions."
     Curr. Opin. Plant Biol. 4:527-532(2001).
     PubMed=11641069
[ 3] Cosgrove D.J., Bedinger P., Durachko D.M.
     "Group I allergens of grass pollen as cell wall-loosening agents."
     Proc. Natl. Acad. Sci. U.S.A. 94:6559-6564(1997).
     PubMed=9177257
[ 4] Yuan S., Wu Y., Cosgrove D.J.
     "A fungal endoglucanase with plant cell wall extension activity."
     Plant Physiol. 127:324-333(2001).
     PubMed=11553760
[ 5] Yennawar N.H., Li L.-C., Dudzinski D.M., Tabuchi A., Cosgrove D.J.
     "Crystal structure and activities of EXPB1 (Zea m 1), a beta-expansin
     and group-1 pollen allergen from maize."
     Proc. Natl. Acad. Sci. U.S.A. 103:14664-14671(2006).
     PubMed=16984999; DOI=10.1073/pnas.0605979103

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