Utilizing HaloTag Technology to Track the Fate of PCSK9 from Intracellular vs. Extracellular Sources

Xi Ai1, *, Paul Fischer2, Oksana C Palyha1, Douglas Wisniewski2, Brian Hubbard1, Karen Akinsanya1, Alison M Strack1, Anka G Ehrhardt2, *
1 Department of Atherosclerosis, Merck Research Laboratories, Rahway, NJ 07065, USA
2 Department of In Vitro Pharmacology, Merck Research Laboratories, Rahway, NJ 07065, USA

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open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

* Address correspondence to these authors at the Department of Atherosclerosis, RY80T-A100, Merck Research Laboratories, 126 E. Lincoln Ave, Rahway, NJ 07065, USA; Tel: 732-594-2286; Fax: 732-594-2510; E-mail:; Department of In Vitro Pharmacology, RY80N-68A, Merck Research Laboratories, 126 E. Lincoln Ave, Rahway, NJ 07065, USA; Tel: 732- 594-5530; Fax: 908-823-3127; E-mail:


The function of a particular protein is dependent upon its localization and milieu. The ability to track the "fate" of a protein is a valuable tool to elucidate its function. We present the use of HaloTag technology to study the localization and fate of human Proprotein Convertase Subtilisin-like Kexin type 9 (PCSK9).

The role of PCSK9 in the regulation of circulating low density lipoprotein-cholesterol (LDL-c) levels is ascribed to binding of circulating PCSK9 to the LDL receptor (LDLR) and subsequent lysosomal degradation of LDLR. However, hints in the literature indicate that intracellular PCSK9 may act on the LDLR, possibly during processing of newly synthesized protein. To address this question, the source and fate of intracellular PCSK9 requires further investigation.

We applied HaloTag technology to distinguish the source of intracellular PCSK9 and showed that newly synthesized intracellular PCSK9 has unique localization from the PCSK9 after re-uptake. This suggests different functions of PCSK9 while interacting with the LDLR.

Keywords: Confocal imaging, HaloTag, intracellular trafficking, LDL receptor, PCSK9, protein label, protein uptake, subcellu-lar localization.