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Published on: July 21, 2021
Proprotein convertases process Pmel17 during secretion
Ralf M Leonhardt1, Nathalie Vigneron, Christoph Rahner
1Department of Immunobiology, Yale University School of Medicine, New Haven, Connecticut 06519, USA. Ralf.Leonhardt@yale.edu
Abstract:
Pmel17 is a melanocyte/melanoma-specific protein that traffics to melanosomes where it forms a fibrillar matrix on which melanin gets deposited. Before being cleaved into smaller fibrillogenic fragments the protein undergoes processing by proprotein convertases, a class of serine proteases that typically recognize the canonical motif RX(R/K)R↓. The current model of Pmel17 maturation states that this processing step occurs in melanosomes, but in light of recent reports this issue has become controversial. We therefore addressed this question by thoroughly assessing the processing kinetics of either wild-type Pmel17 or a secreted soluble Pmel17 derivative. Our results demonstrate clearly that processing of Pmel17 occurs during secretion and that it does not require entry of the protein into the endocytic system. Strikingly, processing proceeds even in the presence of the secretion inhibitor monensin, suggesting that Pmel17 is an exceptionally good substrate. In line with this, we find that newly synthesized surface Pmel17 is already quantitatively cleaved. Moreover, we demonstrate that Pmel17 function is independent of the sequence identity of its unconventional proprotein convertase-cleavage motif that lacks arginine in P4 position. The data alter the current view of Pmel17 maturation and suggest that the multistep processing of Pmel17 begins with an early cleavage during secretion that primes the protein for later functional processing.
Insights
Pmel17 processing occurs during secretion, not in melanosomes, challenging current models of melanocyte protein maturation. This early cleavage primes Pmel17 for its functional role in melanin deposition.
Area of Science:
- Cell Biology
- Biochemistry
- Melanogenesis Research
Background:
- Pmel17 is a key protein in melanocytes and melanoma, forming a matrix for melanin deposition within melanosomes.
- Proprotein convertases (PCs) cleave Pmel17 into smaller fragments, a process critical for its function.
- The exact location of Pmel17 processing has been debated, with the prevailing model suggesting it occurs within melanosomes.
Purpose of the Study:
- To investigate the precise cellular location and kinetics of Pmel17 processing by proprotein convertases.
- To challenge and refine the existing model of Pmel17 maturation and trafficking.
- To determine if Pmel17 processing is dependent on its entry into the endocytic pathway.
Main Methods:
- Comparative analysis of wild-type Pmel17 and a secreted soluble Pmel17 derivative.
- Assessment of processing kinetics under secretion-inhibiting conditions (monensin).
- Evaluation of newly synthesized surface Pmel17 cleavage.
- Analysis of Pmel17's unconventional cleavage motif.
Main Results:
- Pmel17 processing occurs during the secretion pathway, independent of melanosome entry.
- Processing is efficient and occurs even with secretion inhibitors, indicating Pmel17 is a highly effective substrate.
- Newly synthesized surface Pmel17 is already fully cleaved.
- Pmel17 function is not dependent on the specific identity of its cleavage motif, even with an unconventional P4-position sequence.
Conclusions:
- The current model of Pmel17 maturation requires revision; processing initiates early during secretion.
- This early cleavage event primes Pmel17 for subsequent functional processing steps.
- Pmel17 processing is a rapid and efficient event occurring co-translationally or during transit.
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