Lipopolysacchari
des (LPS) of Gram-negative bacteria are important me
diators of bacterialvirulence that can elicit potent en
dotoxic effects. Surfactant protein D (SP-D) shows specific interactionswith LPS, both in vitro an
d in vivo. These interactions involve bin
ding of the carbohy
drate recognition
domain (CRD) to LPS oligosacchari
des (OS); however, little is known about the mechanisms of LPSrecognition. Recombinant neck+CRDs (NCRDs) provi
de an opportunity to
directly correlate bin
dinginteractions with a crystallographic analysis of the bin
ding mechanism. In these stu
dies, we examine
d theinteractions of wil
d-type an
d mutant trimeric NCRDs with rough LPS (R-LPS). Although rat NCRDsboun
d more efficiently than human NCRDs to
Escherichia coli J-5 LPS, both proteins exhibite
d efficientbin
ding to soli
d-phase R
d2-LPS an
d to R
d2-LPS aggregates presente
d in the solution phase. Involvementof resi
dues flanking calcium at the sugar bin
ding site was
demonstrate
d by reciprocal exchange of lysinean
d arginine at position 343 of rat an
d human CRDs. The lectin activity of hNCRDs was inhibite
d byspecific heptoses, inclu
ding
L-
glycero-
-
D-
manno-heptose (
L,
D-heptose), but not by 3-
deoxy-
-
D-
manno-oct-2-ulosonic aci
d (K
do). Crystallographic analysis of the hNCRD
demonstrate
d a novel bin
ding orientationfor
L,
D-heptose, involving the hy
droxyl groups of the si
de chain. Similar bin
ding was observe
d for asynthetic
1
3-linke
d heptose
disacchari
de correspon
ding to heptoses I an
d II of the inner core regionin many LPS. 7-
O-Carbamoyl-
L,
D-heptose an
d D-
glycero-
-
D-
manno-heptose were boun
d via ring hy
droxylgroups. Interactions with the si
de chain of inner core heptoses provi
de a potential mechanism for therecognition of
diverse types of LPS by SP-D.