Fig. 3.

Overview of proposed apelin processing pathways. (A) The previously proposed mechanism for apelin processing [3] suggested that proapelin is cleaved to the longest isoform, apelin-36, and then further processed to shorter isoforms with increased efficacy. (B) Our new model of apelin processing suggests that PCSK3 directly produces apelin-13 from proapelin. The longer apelin-36 and -17 isoforms are putatively produced by other, as yet unidentified endoproteases.