W0434

Extracellular Domain of the LDL Receptor at Endosomal pH. G. Rudenko, L. Henry, R.K. Henderson, K. Ichtchenko, M.S. Brown, J.L. Goldstein, J. Deisenhofer, Biochemistry, UT Southwestern Medical Center, 5323 Harry Hines Blvd. Y4-206, Dallas, TX 75390 USA.

We have solved the three dimensional structure of the extracellular domain of the low density lipoprotein receptor (LDL-R) at endosomal pH to a resolution of 3.7 Å. The structure was solved in a MAD experiment; the asymmetric unit of our crystals contained one protein molecule and 31 tungsten atoms arranged in 2 and a half clusters.

LDL-R mediates cholesterol homeostasis in mammalian cells. LDL-R removes cholesterol laden low density lipoprotein (LDL) from plasma circulation in a process called receptor-mediated endocytosis. Lipoproteins bind the extracellular domain of LDL-R at neutral pH on the cell surface. After internalization, the lipoproteins are released again by the receptors at acidic pH in endocytotic vesicles, enabling the ligands to continue on to the lysosomes for degradation and the free receptor to cycle back to the cell surface.

In the crystal structure of LDL-R at endosomal pH the ligand binding domain (7 cysteine rich repeats) forms a long arc over the rest of the molecule (three EGF-like repeats and a beta-propeller). The modules R4 and R5 of the ligand binding domain, which are crucial for lipoprotein binding, are found associated to the beta-propeller domain via their calcium binding loops. Through this association, the ligand binding domain appears snap shut against the rest of the molecule, preventing lipoprotein binding.

Our structure suggests a mechanism for lipoprotein release in the endosome, whereby the beta-propeller can displace lipoproteins from the ligand binding domain at acidic pH by functioning as an alternate substrate.