The proprotein convertase PC7: unique zymogen activation and trafficking pathways

Area of Science:

• Biochemistry
• Cell Biology
• Molecular Biology

Background:

• The proprotein convertase 7 (PC7) is an ancient, conserved enzyme with unknown zymogen activation and functions.
• Understanding PC7's biosynthesis, localization, and trafficking is crucial for elucidating its biological roles.

Purpose of the Study:

• To characterize the biosynthesis, subcellular localization, and trafficking of membrane-bound rat and human PC7.
• To identify the mechanisms governing PC7's unique transport pathways.

Main Methods:

• Analysis of protein secretion and trafficking using conventional and unconventional pathways.
• Biochemical characterization including sulfation and palmitoylation.
• Genetic manipulation by swapping transmembrane-cytosolic tail (TMCT) domains between PC7 and Furin.
• Subcellular localization studies using electron microscopy in HEK293 cells.

Main Results:

• PC7 prosegment is secreted alone; mature PC7 is partially sulfated and trafficked via conventional and unconventional pathways.
• A fraction of PC7 rapidly transits from the ER to the cell surface via a brefeldin A- and COPII-independent unconventional pathway.
• Palmitoylation of cytosolic cysteines does not influence PC7 trafficking.
• The transmembrane domain of PC7, not the cytosolic tail, directs its sorting to the unconventional secretory pathway.

Conclusions:

• PC7 exhibits distinct zymogen activation, subcellular localization, and trafficking compared to other proprotein convertases.
• PC7's transmembrane domain is a key determinant for its unconventional secretory pathway sorting.
• This study provides novel insights into the complex regulation of PC7 transport and function.