This strategy allowed the prosegment to be successfully expressed and secretedin a yield that was,62-fold higher than bacteria. The advantage of using the Fc portion of the immunoglobulin IgG1 greatly facilitated the expression and secretion of the recombinant protein. Moreover, this extension can also provide additional advantages as proteins fused to Fc regions have improved solubility and stability and can be produced and purified in a large scale using a protein A affinity chromatography. Indeed, most of the successful fusion protein therapeutic approaches today contain different Fc-proteins of immunoglobulins. Herein, we demonstrate for the first time that a chimera containing the prosegmentdirectly binds to pPCSK9and effectively acts as a negative regulator of its ability to induce LDLR degradation. This direct down-regulation of pPCSK9 activity was revealed by intracellular co-expression experiments of Fcpro with either wild type PCSK9 or two of its GOF mutants. The data showed that all PCSK9 forms exhibited reduced activity on LDLR in the presence of Fcpro. This inhibitory effect was also observed to be valid extracellularly, whereupon pre-incubation of pPCSK9 with Fcpro resulted in almost total inactivation of the pPCSK9 ability to induce the degradation of cellular LDLR. Concerning the pPCSK9 region that binds Fcpro, our data suggested that it could implicate the prosegment following the acidic stretch, excluding the C-terminal Gln152. Indeed, earlier data showed that the zymogen proPCSK9 can oligomerize in the ER and that such oligomers can be dissociated using a reducing agent. Subsequent studies revealed that the prosegment alone can also oligomerize in the ER. We further deduce that the acidic region of the prosegmentis not implicated, since the PCSK9 D33-58 is still inhibited by Fcpro. Since the bulky Trp152 is not expected to productively enter the tight catalytic pocket of PCSK9, yet the prosegment Q152W still binds the zymogen, this strongly suggests that the postulated second binding region seemingly does not implicate the catalytic pocket per se, but may be due to either a prosegment;prosegment and/or prosegment;catalytic domain interaction. However, we cannot exclude the possibility that the bulky Fcpro could also modify the catalytic subunit either by direct binding or due to steric hindrance. In that context, overexpression of the prosegment alone with full length PCSK9 resulted in a significant decrease in level of the furin-cleaved form at Arg218Q, revealing that the in trans binding of the overexpressed prosegment allosterically modifies the catalytic subunit in such as way that the cleavage of PCSK9 by furin is largely restricted. Since PCSK9 is now considered a major target for lowering high levels of circulating LDL-cholesterol, which is highly atherogenic and can lead to cardiovascular failure, a number of pharmaceutical companies are developing potent inhibitors of circulating PCSK9 that would prevent its ability to enhance the degradation of liver LDLR. The most BKM120 promising present strategies to inhibit PCSK9 includes the use of blocking monoclonal antibodiesor fibronectin fragmentsthat prevent the formation of the pPCSK9;LDLR LY2835219 complex at the cell surface. Recently, mAbs against PCSK9 that block its interaction with the LDLR have clearly shown very promising results and are now in Phase-II and -III clinical trials. However, such conventional antibodies possess many intrinsic negative characteristics as drugs. In general, they are high-molecular mass proteins, complex to manufacture, and potentially immunogenic; they are unsuited to oral delivery. Above all, the high cost associated with the development and manufacture of mAbs limits their wide applicability to all.