Crystal structure of native and Cd/Cd-substituted Dioclea guianensis seed lectin. A novel manganese-binding site and structural basis of dimer-tetramer association
Da. Wah et al., Crystal structure of native and Cd/Cd-substituted Dioclea guianensis seed lectin. A novel manganese-binding site and structural basis of dimer-tetramer association, J MOL BIOL, 310(4), 2001, pp. 885-894
Diocleinae legume lectins are a group of oligomeric proteins whose subunits
display a high degree of primary structure and tertiary fold conservation
but exhibit considerable diversity in their oligomerisation modes. To eluci
date the structural determinants underlaying Diocleinae lectin oligomerisat
ion, we have determined the crystal structures of native and cadmium-substi
tuted Dioclea guianensis (Dguia) seed lectin. These structures have been so
lved by molecular replacement using concanavalin (ConA) coordinates as the
starting model, and refined against data to 2.0 Angstrom resolution. In the
native (Mn/Ca-Dguia) crystal form (P4(3)2(1)2), the asymmetric unit contai
ns two monomers arranged into a canonical legume lectin dimer, and the tetr
amer is formed with a symmetry-related dimer. In the Cd/Cd-substituted form
(14(1)22), the asymmetric unit is occupied by a monomer. In both crystal f
orms, the tetrameric association is achieved by the corresponding symmetry
operators. Like other legume lectins, native D. guianensis lectin contains
manganese and calcium ions bound in the vicinity of the saccharide-combinin
g site. The architecture of these metal-binding sites (S1 and S2) changed o
nly slightly in the cadmium/cadmium-substituted form. A highly ordered calc
ium (native lec tin) or cadmium (Cd/Cd-substituted lectin) ion is coordinat
ed at the interface between dimers that are not tetrameric partners in a si
milar manner as the previously identified Cd2+ in site S3 of a Cd/Ca-ConA.
An additional Mn2+ coordination site (called S5), whose presence has not be
en reported in crystal structures of any other homologous lectin, is presen
t in both, the Mn/Ca and the Cd/Cd-substituted D, guianensis lectin forms.
On the other hand, comparison of the primary and quaternary crystal structu
res of seed lectins from D. guianensis and Dioclea grandiflora (1DGL) indic
ates that the loop comprising residues 117-123 is ordered to make interdime
r contacts in the D. grandiflora lectin structure, while this loop is disor
dered in the D. guianensis lectin structure. A single amino acid difference
at position 131 (histidine in D. grandiflora and asparagine in D, guianens
is) drastically reduces interdimer contacts, accounting for the disordered
loop. Further, this amino acid change yields a conformation that may explai
n why a pH-dependent dimer-tetramer equilibrium exists for the D. guianensi
s lectin but not for the D. grandiflora lectin. (C) 2001 Academic Press.