Analysis of the polycystin complex (PCC) in human urinary exosome-like vesicles (ELVs).

The polycystin-1 (PC1), polycystin-2 (PC2) and fibrocystin proteins, the respective products of the PKD1, PKD2 and PKHD1 genes, are abundant in urinary exosome-like vesicles (ELVs) where they form the polycystin complex (PCC). ELVs are 100 nm diameter membrane vesicles shed into the urine by the cells lining the nephron. Using MS/MS analysis of ELVs from individuals with PKD1 mutations and controls, we show that in addition to the well-described GPS/GAIN cleavage event in PC1 at 3048 aa and the proprotein convertase cleavage (PPC) event in fibrocystin at 3616 aa, there are multiple other cleavage events in these proteins. The C-terminal 11 transmembrane portion of PC1 undergoes three cleavage events in vivo. The absence of peptides from the C-terminal cytoplasmic tail of fibrocystin implies a cleavage event close to its single TM domain prior to loading onto the ELVs. There is also evidence that the C-terminal tail of PC2 is also cleaved in ELVs. Native gel analysis of the PCC shows that the entire complex is  > 2 MDa in size and that N-terminal GPS/GAIN cleaved PC1 and PPC cleaved fibrocystin ectodomains can be released under non-reducing conditions and resolve at 300 kDa. This paper shows that the three major human cystogene proteins are detectable in human urinary ELVs and that all three undergo post-translational proteolytic processing. Human urinary ELVs may be a useful source of material in the search for proteins that interact with the PCC.

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