Isolation and characterization of the circulating truncated form of PCSK9

Abstract

Proprotein convertase subtilisin-kexin type 9 (PCSK9) is a secreted protein which regulates serum LDL cholesterol. It circulates in human and rodent serum in an intact form and a major truncated form. Previous in vitro studies involving the expression of human PCSK9 genetic variants and in vivo studies of furin knockout mice suggest that the truncated form is a furin cleavage product. However, the circulating truncated form of PCSK9 has not been isolated and characterized. Utilizing antibodies which bind to either the catalytic domain or the C-terminal domain of PCSK9, the truncated PCSK9 was isolated from serum. MS was used to determine that this form of PCSK9 is a product of in vivo cleavage at Arg218 resulting in pyroglutamic acid formation of the nascent N terminus corresponding to Gln219 of intact PCSK9. We also determined that the truncated PCSK9 in serum lacked the N-terminal segment which contains amino acids critical for LDL receptor binding. A truncated PCSK9, expressed and purifi ed from HEK293 cells with identical composition as the circulating truncated protein, was not active in inhibition of LDL uptake by HepG2 cells. These studies provide a defi nitive characterization of the composition and activity of the truncated form of PCSK9 found in human serum. -Han, B., P. I. Eacho, M. D. Knierman, J. S. Troutt, R. J. Konrad, X. Yu, and K. M. Schroeder. Isolation and characterization of the circulating truncated form of PCSK9. J. Lipid Res. 2014. 55: 1505 - 1514 . © 2014 by the American Society for Biochemistry and Molecular Biology, Inc.