The carbohydrate recognition domain of collectins

Collectins are effector molecules of the innate immune system that play an
important role in the first line of defence against bacteria, viruses and
fungi. Most of their interactions with microorganisms are mediated
through their carbohydrate recognition domain (CRD), which binds in a
Ca2+-dependent manner to glycoconjugates. This domain is a well-known
structure that is present in a larger group of proteins comprising the C-type
lectin domain family. Collectins form a subgroup within this family based
on the presence of a collagen domain and the trimerization of CRDs,
which are essential for the ligand-binding properties of these proteins. The
ligand specificity among the nine collectin members is significantly different
as a result of both the structural organization of the trimers and specific
sequence changes in the binding pocket of the CRD. In addition, some collectin
members have additional features, such as N-linked glycosylation of
CRD residues and additional loop structures within the CRD that have a
large impact on their interaction with the glycoconjugates present on
microorganisms or host cells. The availability of crystal structures of three
members of the collectin family (surfactant proteins A and D and mannanbinding
protein) provides an important tool for addressing the impact of
these CRD differences on ligand binding. In this review, the structural differences
and similarities between the CRDs of collectins are summarized
and their relationship with their ligand-binding characteristics is discussed.