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The autosomal recessive hypercholesterolemia (ARH) protein interfaces directly with the clathrin-coat machinery
Sanjay K Mishra1, Simon C Watkins, Linton M Traub
1Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261 USA.
Insights
Autosomal recessive hypercholesterolemia (ARH) protein binds directly to clathrin and phosphoinositides, impacting LDL receptor traffic. This defective sorting adaptor function in hepatocytes leads to faulty LDL receptor transport and hypercholesterolemia.
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- Low-density lipoprotein (LDL) receptor is crucial for cholesterol metabolism.
- Mutations in LDL receptor or apolipoprotein B cause elevated plasma cholesterol and atherosclerosis.
- Autosomal recessive hypercholesterolemia (ARH) is a rare genetic disorder with similar phenotypes, linked to mutations in the ARH gene.
Purpose of the Study:
- To investigate the molecular mechanism of the ARH protein.
- To determine how ARH mutations lead to hypercholesterolemia.
Main Methods:
- Studied ARH protein interactions with clathrin and phosphoinositides.
- Utilized HeLa cells to observe ARH colocalization with endocytic proteins.
- Tracked LDL receptor trafficking in relation to ARH.
Main Results:
- ARH binds directly to soluble clathrin trimers and clathrin adaptors via its beta subunit appendage domain.
- ARH also binds directly to phosphoinositides, regulating clathrin bud assembly.
- In HeLa cells, ARH colocalizes with endocytic proteins, and LDL receptor passes through ARH-positive sites before reaching early endosomes.
Conclusions:
- ARH acts as a sorting adaptor, crucial for proper LDL receptor traffic.
- Defective ARH function in hepatocytes impairs LDL receptor sorting, leading to hypercholesterolemia.
- Findings elucidate the molecular basis of ARH and offer potential therapeutic targets.
Abstract:
The low density lipoprotein (LDL) receptor plays a pivotal role in cholesterol metabolism. Inherited mutations that disturb the activity of the receptor lead to elevations in plasma cholesterol levels and early-onset coronary atherosclerosis. Defects in either the LDL receptor or apolipoprotein B, the proteinaceous component of LDL particles that binds the LDL receptor, elevate circulating LDL-cholesterol levels in an autosomal-dominant fashion, with heterozygotes displaying values between homozygous and normal individuals. Rarely, similar clinical phenotypes occur with a recessive pattern of inheritance, and several genetic lesions in the autosomal recessive hypercholesterolemia (ARH) gene on chromosome 1 have been mapped in this class of patients. ARH has an N-terminal phosphotyrosine-binding (PTB) domain evolutionarily related to that found in Disabled-2 and numb, two endocytic proteins. PTB domains bind to the consensus sequence FXNPXY, corresponding to the internalization motif of the LDL receptor. We show here that in addition to the FXNPXY sequence, ARH binds directly to soluble clathrin trimers and to clathrin adaptors by a mode involving the independently folded appendage domain of the beta subunit. At steady state, ARH colocalizes with endocytic proteins in HeLa cells, and the LDL receptor fluxes through peripheral ARH-positive sites before delivery to early endosomes. Because ARH also binds directly to phosphoinositides, which regulate clathrin bud assembly at the cell surface, our data suggest that in ARH patients, defective sorting adaptor function in hepatocytes leads to faulty LDL receptor traffic and hypercholesterolemia.