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Pacifastin, a novel 155-kDa heterodimeric proteinase inhibitor containing a unique transferrin chain
Z Liang1, L Sottrup-Jensen, A Aspán
1Department of Physiological Botany, Uppsala University, Villavägen 6, S-752 36 Uppsala, Sweden.
Abstract:
A 155-kDa proteinase inhibitor, pacifastin, from plasma of the freshwater crayfish, Pacifastacus leniusculus, was found to be composed of two covalently linked subunits. The two subunits are encoded by two different mRNAs, which were cloned and sequenced. The heavy chain of pacifastin (105 kDa) is related to transferrins, containing three transferrin lobes, two of which seem to be active for iron binding. The light chain of pacifastin (44 kDa) is the inhibitory subunit, and has nine cysteine-rich inhibitory domains that are homologous to each other and to low molecular weight proteinase inhibitors isolated from the grasshopper, Locusta migratoria. The nine light chain domains and the Locusta inhibitors share a characteristic cysteine array (Cys-Xaa9-12-Cys-Xaa2-Cys-Xaa-Cys-Xaa6-8-Cys-Xaa4++ +-Cys) distinct from any described proteinase inhibitor family, suggesting that they constitute a new family of proteinase inhibitors. Pacifastin is the first known protein that has combined properties of a transferrin-like molecule and a proteinase inhibitor.
Insights
Pacifastin, a crayfish protein, features a heavy chain similar to transferrins and a light chain with unique inhibitory domains. This discovery reveals a novel proteinase inhibitor family with dual functionality.
Area of Science:
- Biochemistry
- Molecular Biology
- Immunology
Background:
- Pacifastin is a 155-kDa proteinase inhibitor found in the plasma of the freshwater crayfish, Pacifastacus leniusculus.
- It is composed of two covalently linked subunits, a heavy chain (105 kDa) and a light chain (44 kDa).
Purpose of the Study:
- To elucidate the molecular composition and functional domains of pacifastin.
- To characterize the novel proteinase inhibitor family suggested by the light chain's structure.
Main Methods:
- Cloning and sequencing of the two distinct mRNAs encoding the pacifastin subunits.
- Bioinformatic analysis to identify homologous domains and conserved cysteine arrays.
Main Results:
- The heavy chain is related to transferrins, with potential iron-binding activity.
- The light chain contains nine unique cysteine-rich inhibitory domains, forming a new proteinase inhibitor family.
- Pacifastin exhibits a unique combination of transferrin-like and proteinase inhibitory properties.
Conclusions:
- Pacifastin represents a novel class of proteins with dual transferrin-like and proteinase inhibitory functions.
- The identified light chain domains constitute a new family of proteinase inhibitors, distinct from previously described families.
- This finding expands our understanding of protein diversity and evolutionary mechanisms in invertebrates.