Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the extracellular 
domain of the low density lipoprotein receptor (LDLR) at the cell surface, and 
disrupts the normal recycling of the LDLR. However, the exact mechanism by which 
the LDLR is re-routed for lysosomal degradation remains to be determined. To 
clarify the role of the cytoplasmic domain of the LDLR for re-routing to the 
lysosomes, we have studied the ability of PCSK9 to degrade a chimeric receptor 
which contains the extracellular and transmembrane domains of the LDLR and the 
cytoplasmic domain of the transferrin receptor. These studies were performed in 
CHO T-REx cells stably transfected with a plasmid encoding the chimeric receptor 
and a novel assay was developed to study the effect of PCSK9 on the LDLR in 
these cells. Localization, function and stability of the chimeric receptor were 
similar to that of the wild-type LDLR. The addition of purified gain-of-function 
mutant D374Y-PCSK9 to the culture medium of stably transfected CHO T-REx cells 
showed that the chimeric receptor was degraded, albeit to a lower extent than 
the wild-type LDLR. In addition, a mutant LDLR, which has the three lysines in 
the intracellular domain substituted with arginines, was also degraded by 
D374Y-PCSK9. Thus, the mechanism for the PCSK9-mediated degradation of the LDLR 
does not appear to involve an interaction between the endosomal sorting 
machinery and LDLR-specific motifs in the cytoplasmic domain. Moreover, 
ubiquitination of lysines in the cytoplasmic domain does not appear to play a 
critical role in the PCSK9-mediated degradation of the LDLR.