Characterization of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Trafficking Reveals a Novel Lysosomal Targeting Mechanism via Amyloid Precursor-like Protein 2 (APLP2)

Hong Liang,David L. Shelton,Rachel M. DeVay

doi:10.1074/jbc.m113.453373

Abstract

Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates low density lipoprotein receptor protein levels by diverting it to lysosomes. Monoclonal antibody therapeutics aimed to neutralize PCSK9 have been shown to successfully lower serum LDL levels; however, we previously found that such therapeutic antibodies are subject to PCSK9-mediated clearance. In this study, we discovered that PCSK9 interacts via its C-terminal domain directly and in a pH-dependent manner with amyloid precursor protein as well as its closely related family member, amyloid precursor protein-like protein 2. Furthermore, we determined that amyloid precursor protein-like protein-2, but not amyloid precursor protein, is involved in mediating postendocytic delivery of PCSK9 to lysosomes and is therefore important for PCSK9 function. Based on our data, we propose a model for a lysosomal transport complex by which a soluble protein can target another protein for degradation from the luminal side of the membrane by bridging it to a lysosomally targeted transmembrane protein.

Highlights

proprotein convertase subtilisin/kexin type 9 (PCSK9) functions to degrade the low density lipoprotein receptor (LDLR) by a previously unknown lysosomal sorting mechanism
LDLR clusters on the cell surface were strongly colocalized with amyloid precursorlike protein 2 (APLP2) cell surface clusters with an average icorr value of 0.83 Ϯ 0.20 when in the presence of PCSK9 (Fig. 7A). These results suggest that PCSK9 and LDLR are spatially distributed on the cell surface with APLP2 and indicate that the three proteins may be internalized in the same endocytic compartments
The reported blocking of PCSK9 internalization by EGF-AB peptide that seemingly contradicts our observations [17] could be due to direct binding of EGF-AB peptides to other LDLR-interacting proteins that in turn could affect processes such as receptor clustering that may be required for PCSK9 endocytosis

Summary

Background

PCSK9 functions to degrade the LDLR by a previously unknown lysosomal sorting mechanism. High LDL serum levels correlate strongly with atherosclerosis and cardiovascular heart disease; loss of function LDLR mutants were identified as determining factors of autosomal dominant familial hypercholesterolemia [5] Another protein that has been causally linked to familial hypercholesterolemia is proprotein convertase subtilisin/kexin type 9 (PCSK9) [7,8,9]. After PCSK9 binds to LDLR on the cell surface, the complex is internalized, and their interaction tightens as pH levels drop along the endolysosomal route by which PCSK9 mediates LDLR degradation [13,14,15,16]. We sought to better characterize PCSK9 trafficking and in doing so revealed that PCSK9 can divert LDLR to lysosomes from the luminal side of the membrane via a novel lysosomal transport complex

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION