Ldl

receptor

Ligand-binding arm (R1-R7;

ß-propeller domain

NPXY sorting signal

Cholesterol esters

Phospholipid monolayer

ApoB protein

Cholesterol esters

Phospholipid monolayer

ApoB protein

LDL particle

▲ FIGURE 17-30 Model for pH-dependent binding of LDL particles by the LDL receptor. Schematic depiction of LDL receptor at neutral pH found at the cell surface (left) and at acidic pH found in the interior of the late endosome (right). At the cell surface, apoB-100 on the surface of a LDL particle binds tightly to the receptor. Of the seven repeats (R1-R7) in the ligand-binding arm, R4 and R5 appear to be most critical for LDL binding. Within the endosome, histidine residues in the p-propeller domain of the LDL receptor become protonated. The positively charged propeller can bind with high affinity to the ligand-binding arm, which contains negatively charged residues, causing release of the LDL particle. Experimental electron density and C„ trace model of the extracellular region of the LDL receptor at pH 5.3 based on X-ray crystallographic analysis. In this conformation, extensive hydrophobic and ionic interactions occur between the p propeller and the R4 and R5 repeats. Red spheres represent Ca2+ ions. [Part (b) from G. Rudenko et al., 2002, Science 298:2353.]

Surface of P-propeller domain becomes positively charged, and then binds to the ligand-binding arm

P-propeller domain

Released LDL particle

Released LDL particle

P-propeller domain

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