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Fig. 1. The LDL receptor family. The structural organization of members of
the LDL receptor family. Their extracellular domains contain clusters of
complement-type repeats that are the site of ligand binding, and also
ß-propellers, which are essential for the pH-dependent release of ligands
in endosomes. The cytoplasmic tails harbor recognition sites for adaptor
proteins that are involved in protein trafficking and signal transduction. All
ectodomains share significant sequence similarity in line with the ability of
the receptors to bind apolipoproteins. By contrast, the cytoplasmic domains
are unique, indicating distinct cellular fates for ligands internalized by
individual receptors. Receptors on the left are considered to be core members
of the protein family as their extracellular domains are built from a unifying
module of amino-terminal complement-type repeats, followed by a
carboxyl-terminal cluster of ß-propellers and epidermal growth
factor-type repeats. This module can exist in single (e.g. LDLR) or multiple
(e.g. LRP2) copies in the receptors. Receptors on the right are more distantly
related, as the module is inverted (LRP5/6) or combined with motifs that are
not seen in the other receptors (e.g. SORLA). APOER2, apolipoprotein E
receptor 2; Ce, C. elegans; LDLR, low-density lipoprotein receptor;
LRP, LDL receptor-related protein; MEGF7, multiple epidermal growth
factor-type repeat containing protein 7; RME-2, receptor-mediated
endocytosis-2; SORLA, sortilin-related receptor with A-type repeats; VLDLR,
very low-density lipoprotein receptor.
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