In plants, oxysterols brassinosteroids (BR) and ecdysteroids (ES) can be found. BR act as plant hormones with a positive effect on growth, stress tolerance or senescence. Their mechanism of action is based on signal transduction via a membrane receptor cascade, with or without assistance of oxysterol-binding proteins. The role of ES as insect hormones in plants is unknown. ES are likely to show an antifeedant effect on herbivorous pests, but other as yet unknown roles in plants are not excluded. A range of affinity carriers with immobilized synthetic analogues of plant BS and ES were prepared. Using these carriers, some proteins with affinity to the oxysterols were separated from cytosol extracts of plants. The proteins were assessed by electrophoresis and identified by sequencing. One of them is an osmotin-like protein precursor known as a pathogenesis- related protein. Another protein, RuBisCO, is known as the key enzyme in the Calvine cycle of the dark phase of photosynthesis. The interactions of these proteins with the oxysterols remain unclear.
Filamentous fungi produce and secrete beta-N-acetylhexosaminidases, Hex, as important components of the binary chitinolytic systems involved in the formation of septa and hyphenation. Enzyme reconstitution experiments published previously indicate that Hex can occur in the form of two molecular species containing either one or two molecules of the propeptide noncovalently associated with the enzyme dimer. Here, we describe a novel mechanism for the regulation of the activity of Hex based on the association of their catalytic subunits with the large N-terminal propeptides in vivo. We show that the enzyme precursor is processed early in the biosynthesis, shortly after the addition of N-glycans through the action of a dibasic peptidase, cleaving both before and after the dibasic sequence. The processing site for this unique dibasic peptidase, different from that of kexins, is conserved among the beta-N-acetylhexosaminidases from filamentous fungi, and inhibition of the dibasic peptidase abrogates enzyme folding and activation. Binding of the released propeptide to the catalytic subunit of Hex is essential for its activation. An examination of the kinetics of Hex activation and dimerization in vitro allowed us to understand the unusually high efficiency of the assembly of this enzyme. We also report that the fungus is able to actively regulate the concentration of the processed propeptide in endoplasmic reticulum and thus the specific activity of the produced Hex. This novel regulatory mechanism enables the control of the catalytic activity and architecture of the secreted enzyme according to the needs of the producing cell at various stages of its growth cycle.
- MeSH
- acetylglukosamin analogy a deriváty farmakologie MeSH
- aktivace enzymů MeSH
- beta-N-acetylhexosaminidasy metabolismus sekrece MeSH
- biologický transport MeSH
- dimerizace MeSH
- endoplazmatické retikulum metabolismus MeSH
- financování organizované MeSH
- furin metabolismus MeSH
- genetická transkripce imunologie MeSH
- houby enzymologie MeSH
- katalýza MeSH
- molekulární sekvence - údaje MeSH
- prekurzory enzymů metabolismus sekrece MeSH
- sekvence aminokyselin MeSH
- sekvenční homologie aminokyselin MeSH
- stabilita enzymů MeSH
- thiazoly farmakologie MeSH
