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MIF protein are theta-class glutathione S-transferase homologs
F A Blocki1, L B Ellis, L P Wackett
1Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul 55108.
Abstract:
MIF proteins are mammalian polypeptides of approximately 13,000 molecular weight. This class includes human macrophage migration inhibitory factor (MIF), a rat liver protein that has glutathione S-transferase (GST) activity (TRANSMIF), and the mouse delayed early response gene 6 (DER6) protein. MIF proteins were previously linked to GSTs by demonstrating transferase activity and observing N-terminal sequence homology with a mu-class GST (Blocki, F.A., Schlievert, P.M., & Wackett, L.P., 1992, Nature 360, 269-270). In this study, MIF proteins are shown to be structurally related to the theta class of GSTs. This is established in three ways. First, unique primary sequence patterns are developed for each of the GST gene classes. The patterns identify the three MIF proteins as theta-like transferase homologs. Second, pattern analysis indicates that GST members of the theta class contain a serine residue in place of the N-terminal tyrosine that is implicated in glutathione deprotonation and activation in GSTs of known structure (Liu, S., et al., 1992, J. Biol. Chem. 267, 4296-4299). The MIF proteins contain a threonine at this position. Third, polyclonal antibodies raised against recombinant human MIF cross-react on Western blots with rat theta GST but not with alpha and mu GSTs. That MIF proteins have glutathione-binding ability may provide a common structural key toward understanding the varied functions of this widely distributed emerging gene family. Because theta is thought to be the most ancient evolutionary GST class, MIF proteins may have diverged early in evolution but retained a glutathione-binding domain.
Insights
Macrophage migration inhibitory factor (MIF) proteins are structurally related to the ancient theta class of glutathione S-transferases (GSTs). This finding suggests MIF proteins may have retained a glutathione-binding domain throughout evolution.
Area of Science:
- Biochemistry
- Molecular Biology
- Evolutionary Biology
Background:
- Macrophage migration inhibitory factor (MIF) proteins are mammalian polypeptides with diverse functions.
- Previous studies linked MIF proteins to glutathione S-transferases (GSTs) based on transferase activity and sequence homology.
- The structural classification of MIF proteins within the GST superfamily remained unclear.
Purpose of the Study:
- To determine the structural relationship of MIF proteins to different classes of glutathione S-transferases (GSTs).
- To investigate the evolutionary divergence and functional implications of MIF proteins within the GST family.
Main Methods:
- Development of unique primary sequence patterns for each GST gene class.
- Pattern analysis to classify MIF proteins (human MIF, rat TRANSMIF, mouse DER6) within the GST superfamily.
- Analysis of N-terminal residues critical for glutathione binding and activation.
- Cross-reactivity testing using polyclonal antibodies against human MIF and various GST classes (theta, alpha, mu) via Western blots.
Main Results:
- MIF proteins were identified as theta-like transferase homologs based on unique sequence patterns.
- Unlike known GSTs, MIF proteins possess a threonine residue at the N-terminal position critical for glutathione activation, whereas theta GSTs have a serine.
- Polyclonal antibodies against human MIF showed cross-reactivity with rat theta GST but not with alpha or mu GSTs, supporting a structural similarity to theta GSTs.
- Evidence suggests MIF proteins possess glutathione-binding ability.
Conclusions:
- MIF proteins are structurally related to the theta class of GSTs, suggesting an ancient evolutionary origin.
- The conserved glutathione-binding domain in MIF proteins may explain their varied functions.
- MIF proteins likely diverged early in evolution while retaining key functional domains.