# A non-canonical fungal peroxisome PTS-1 signal, SYM, and its evolutionary aspects

## TL;DR

This study shows that a non-canonical plant signal, SYM, can target proteins to peroxisomes in fungi, suggesting evolutionary flexibility in peroxisomal protein localization.

## Contribution

The discovery that SYM, a non-canonical PTS-1 signal, functions in fungi and may contribute to protein adaptability through partial peroxisomal localization.

## Key findings

- SYM, a non-canonical plant PTS-1 signal, localizes to peroxisomes in Aspergillus nidulans when fused to GFP.
- In silico analysis of 1,010 fungal genomes reveals diverse SYM-proteins with variable functions and predicted non-peroxisomal localization.
- Fungal pectinesterases frequently contain SYM signals, suggesting convergent evolution of partial peroxisomal localization.

## Abstract

Proteins localized to peroxisomes, particularly those expressed under specific conditions or in low abundance, are often undetected by routine proteomics methods due to detection sensitivity limits. In silico identification and experimental validation of peroxisomal targeting signals (PTSs) offer a reliable alternative. We demonstrate that SYM, a non-canonical plant PTS-1 signal, functions similarly in Aspergillus nidulans, as GFP tagged with a SYM C-terminal tripeptide localizes to peroxisomes. One of two native A. nidulans proteins with C-terminal SYM tripeptide shows weak peroxisomal localization alongside cytoplasmic presence, indicating that only a subset of proteins with non-canonical signals access peroxisomes. In silico analysis of 1,010 fungal genomes identified diverse SYM-proteins with variable functions, suggesting that non-canonical PTS-1 signals may evolve spontaneously. Two-thirds of SYM-proteins are predicted to localize to specific intracellular compartments other than the peroxisome. We propose that despite their predicted localization, these proteins possessing SYM as a non-canonical peroxisomal signal might also have peroxisomal presence. Among SYM-proteins, pectinesterases, known plant pathogen virulence factors, were frequent. Notably, 25% of fungal pectinesterases harbor non-canonical PTS-1 signals, suggesting that partial peroxisomal localization of pectinesterases has evolved convergently. This suggests that partial peroxisomal localization may enhance protein functional flexibility, contributing to the organism’s adaptability.

The online version contains supplementary material available at 10.1038/s41598-025-13871-x.