Method for detecting autoprocessed, secreted PCSK9

A method for identifying processed, secreted proprotein convertase subtilisin-kexin type 9 (“PCSK9”) to the exclusion of nonprocessed, secreted PCSK9 in cell supernatant, which comprises:

• (a) collecting cell supernatant from an individual cell transfected with an expression construct comprising nucleic acid encoding PCSK9 wherein an epitope tag is inserted immediately subsequent to nucleic acid encoding residue Q in the stretch of amino acids FAQ, said series of amino acids corresponding to amino acid residues 150-152 in human PCSK9, amino acid residues 153-155 in murine PCSK9, or amino acid residues 149-151 in rat PCSK9;
• (b) introducing into the cell supernatant an antibody or anti-epitope molecule capable of recognizing the epitope tag only when located at the extreme amino-terminus, said antibody or anti-epitope molecule carrying a selectable marker which can be detected;
• and
• (c) detecting bound antibody or anti-epitope molecule carrying said selectable marker, bound antibody or anti-epitope molecule indicating processed, secreted PCSK9.