title: PCSK9 is phosphorylated by a Golgi casein kinase-like kinase ex vivo and circulates as a phosphoprotein in humans.
abstract: Proprotein convertase subtilisin/kexin 9 (PCSK9) is a secreted glycoprotein that regulates the degradation of the low-density lipoprotein receptor. Single nucleotide polymorphisms in its gene associate with both hypercholesterolemia and hypocholesterolemia, and studies have shown a significant reduction in the risk of coronary heart disease for 'loss-of-function' PCSK9 carriers. Previously, we reported that proPCSK9 undergoes autocatalytic processing of its prodomain in the endoplasmic reticulum and that its inhibitory prosegment remains associated with it following secretion. Herein, we used a combination of mass spectrometry and radiolabeling to report that PCSK9 is phosphorylated at two sites: Ser47 in its propeptide and Ser688 in its C-terminal domain. Site-directed mutagenesis suggested that a Golgi casein kinase-like kinase is responsible for PCSK9 phosphorylation, based on the consensus site, SXE/S(p). PCSK9 phosphorylation was cell-type specific and occurs physiologically because human plasma PCSK9 is phosphorylated. Interestingly, we show that the naturally occurring 'loss-of-function' variant PCSK9(R46L) exhibits significantly decreased propeptide phosphorylation in the Huh7 liver cell line by 34% (P < 0.0001). PCSK9(R46L) and the engineered, unphosphorylated variant PCSK9(E49A) are cleaved following Ser47, suggesting that phosphorylation protects the propeptide against proteolysis. Phosphorylation may therefore play an important regulatory role in PCSK9 function. These findings will be important for the future design of PCSK9 inhibitors.
authors.affiliation: Chronic Disease Program, Ottawa Health Research Institute, The Ottawa Hospital, Ottawa, Ontario, Canada.
authors.forename: Thilina
authors.initials: T
authors.lastname: Dewpura
authors.forename: Angela
authors.initials: A
authors.lastname: Raymond
authors.forename: Josée
authors.initials: J
authors.lastname: Hamelin
authors.forename: Nabil G
authors.initials: NG
authors.lastname: Seidah
authors.forename: Majambu
authors.initials: M
authors.lastname: Mbikay
authors.forename: Michel
authors.initials: M
authors.lastname: Chrétien
authors.forename: Janice
authors.initials: J
authors.lastname: Mayne
mesh_terms: D000818:Animals; D047388:Casein Kinases / Q000378:metabolism*; D002460:Cell Line; D006224:Cricetinae; D018076:DNA, Complementary / Q000378:metabolism; D006056:Golgi Apparatus / Q000201:enzymology / Q000378:metabolism*; D006801:Humans; D006937:Hypercholesterolemia / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D013058:Mass Spectrometry / Q000379:methods; D016297:Mutagenesis, Site-Directed; D010455:Peptides / Q000737:chemistry; D010750:Phosphoproteins / Q000737:chemistry*; D010766:Phosphorylation; D000071449:Proprotein Convertase 9; D043484:Proprotein Convertases; D012697:Serine Endopeptidases / Q000737:chemistry / Q000502:physiology*
chemical_list: D018076:DNA, Complementary; D010455:Peptides; D010750:Phosphoproteins; D047388:Casein Kinases; C472125:PCSK9 protein, human; D000071449:Proprotein Convertase 9; D043484:Proprotein Convertases; D012697:Serine Endopeptidases