Iron limitation and its effect on membrane proteins and siderophore transport in Neurospora crassa

Cells of the fungus Neurospora crassa were grown under iron-deficient and iron-sufficient conditions and their plasma membrane proteins were compared. Three strains were studied: N. crassa 74A (wild type), a siderophore-free mutant N. crassa (arg-5 ota aga) as well as a 'slime' variant of N. crassa which lacks a cell wall. Plasma membranes were purified, solubilized and analyzed by one-dimensional SDS/polyacrylamide gel electrophoresis yielding approximately 50 distinct protein bands with molecular masses in the range 14-160 kDa. Iron-sufficient and iron-deficient growth resulted in nearly identical plasma membrane protein profiles in all strains. Although minor alterations in the proportion of certain proteins could be detected, significant overproduction of certain membrane proteins during iron limitation could not be observed. Transport of 55Fe-labeled siderophores seems to be correlated to the degree of iron limitation. For example, transport rates were enhanced fivefold after 16 h of growth in iron-deficient medium compared to growth in iron-sufficient medium. Extraction and HPLC measurement of siderophores from conidiospores yielded approximately 10(-15) mol/spore, indicating that germination tubes and young cells used for transport measurements are not iron-deficient. It is suggested that the putative transport systems for siderophores in fungal plasma membranes are constitutively expressed and enhanced uptake of siderophores during iron limitation is rather the result of cellular transport regulation mechanisms.