Potential biological roles for two novel human glycan-binding proteins have been
investigated.
In the first part of the work, analysis of the scavenger receptor C-type lectin
(SRCL) leads to the proposal that the primary function of this receptor is to clear
glycoproteins released from neutrophils. The expression of SRCL was characterized by
screening of cDNA libraries and by immunostaining of tissues. These results confirm and
extend earlier reports that SRCL is found in a range of tissues and is expressed by
cultured umbilical vein endothelial cells, and demonstrated that SRCL is localized
primarily in endothelial cells. SRCL is thus positioned to interact with circulating
glycoproteins and cells. Potential ligands were isolated from neutrophils by affinity
chromatography and shown by mass spectrometry to comprise lactoferrin and other
soluble granule proteins. The interaction between SRCL and lactoferrin was found to be
dependent on fucose residues and analysis of lactoferrin N-glycans by mass spectrometry
revealed multiple Lewisx epitopes borne on unsialylated triantennary glycans, consistent
with selective binding of SRCL to the Lewisx trisaccharide. Transfected cells expressing
SRCL were shown to internalize fluorescein-labelled lactoferrin. Taken together, these
results provide support for the hypothesis that soluble glycoproteins in neutrophil
secondary granules are tagged with the Lewisx epitope to direct their clearance by SRCL
after release at sites of inflammation.
Prolectin, a novel C-type lectin, was identified by screening of the human genome.
Screening of cDNA libraries and immunostaining were used to ascertain that prolectin
expression is restricted to dividing B cells in and around germinal centres of lymphoid
tissues. This expression pattern, together with the presence of motifs in the cytoplasmic
tail of prolectin that may interact with intracellular signalling molecules, leads to the
suggestion that prolectin may have a role in the migration of B cells during the adaptive
immune response.