Summary:
Peroxisomes are highly metabolic organelles involved in key functions in eukaryotic
cells. One of them is the well-studied purine assimilation pathway in Aspergillus
nidulans, where part of the purine catabolic enzymes is located inside peroxisomes.
Since certain steps of the pathway occur inside these organelles, while others in the
cytoplasm, there needs to be a transport of purine metabolites across the peroxisomal
membrane. As these intermediates are unable to diffuse freely through lipid bilayers,
specific transporters must exist for their passage membrane across the peroxisomal
membrane. In order to identify them, we conducted an in silico search for evolutionarily
correlated gene loss associated with fungal peroxisomal uric acid oxidase (UOX), where
PMP22-like proteins, found to be the most promising candidates, as some of them
function as promiscuous channels in organellar membranes. To investigate whether
PMP22 channels actually have a role in peroxisomal uric acid transport and catabolism,
we functionally analyzed the relevant Aspergillus nidulans protein, WscA, which is
encoded by the uncharacterized gene AN7258. We confirmed that WscA is a
peroxisomal membrane protein that co-localizes with PTS1-tagged mRFP, UOX or
HexA, the latter considered a protein of Woronin bodies (WBs), organelles originating
from peroxisomes that dynamically plug septal pores in filamentous ascomycetes. Our
results suggest that in A. nidulans, unlike some other ascomycetes, there is no strict
protein segregation of peroxisomal and WB-specific proteins. Importantly, genetic
deletion of wscA, but not of hexA, led to lack of peroxisomal localization at septal pores,
suggesting that WscA is a key factor for septal pore functioning. Additionally, ΔwscA
resulted in increased sensitivity to oxidative stress, apparently as a consequence of not
only the inability for septal pore plugging, but also a recorded reduction in peroxisome
biogenesis. However, deleting wscA had no effect on uric acid or purine utilization, as
we hypothesized, a result in also line with the observation that expression of WscA was
not affected by regulatory mutants and conditions known to control purine catabolic
enzymes. Our results are discussed within the frame of previous studies of WscA
homologues in other fungi, as well as, the observed gene losses of PMP22 and
peroxisomal uric acid oxidase.
Keywords:
filamentous fungi, uric acid, urate oxidase, Woronin bodies, septa
