HIT family

The histidine triad (HIT) family of proteins forms a ubiquitous superfamily of small cytosolic, nucleotide binding proteins. The distinguishing feature of these enzymes is an active site motif composed of the sequence His-X-His-X-His-XX, where X is a hydrophobic amino acid (Figure 2).  HIT proteins are phosphoramidases, dinucleotide hydrolyases and nucleotidylyl transferases. These proteins have been highly conserved throughout evolution and are present in all kingdoms of life. The human genome encodes for 7 HIT proteins, which can be classified into 5 groups (figure 3):

        Figure 2 - Hint2 -Structural characterization PDB_4INC
        - HIT motif - dimeric binding domain links
        Helices in pink, Beta- sheets in yellow

 

1. Hint proteins (HIstidine triad NucleoTide-binding proteins)

Hint proteins constitute the most ancient and conserved HIT proteins, with at least one representative found in all fully sequenced genomes. The human genome contains three different HINT genes that code for HINT1, HINT2 and HINT3.

Publications :
Disruption of tumor suppressor gene Hint1 leads to remodeling of the lipid metabolic phenotype of mouse liver - Beyoğlu D et al., 2014 - J. Lipid Res.

Characterization of the effect of the mitochondrial protein Hint2 on intracellular Ca(2+) dynamics - Ndiaye D. et al., 2013 - Biophys J.

Disruption of the histidine triad nucleotide-binding hint2 gene in mice affects glycemic control and mitochondrial function- Martin J et al., 2013 - Hepatology
 

• Hint1. Hint1 forms homodimers, with each subunit binding a nucleotide. Hint1 catalyses the hydrolysis of nucleosides phosphoraminidate and acyl-adenylates but has a preference for purine phosphoramidates over pyrimidine phosphoramidates. Endogenous substrates for Hint1 might be aminoacyl-AMP intermediates. Aminoacyl-tRNA synthetases charge aminoacids with their corresponding cognate tRNAs to form aminoacyl-tRNAs. The reaction proceeds by the formation of an aminoacyl-AMP intermediate. Hint1 knockout mice (Hint1^-/)^- mice develop normally, but present an enhanced susceptibility to carcinogens. We found that Hint1^-/- mice are resistant to hepatic ischemia/reperfusion injury. 
• Hint2. We have reported that Hint2 is located in mitochondria and that HINT2 mRNA is significantly downregulated in several human carcinomas. The characterization of our Hint2 knockout mice (Hint2^-/-) mice revealed a striking metabolic phenotype.  Major changes in Hint2^-/- mice were the accumulation of hepatic triglycerides, associated with a decrease in the activity of the β-oxidation enzyme short chain 3-hydroxyacyl-CoA dehydrogenase, and an impaired glucose tolerance.
• Hint3. A third HINT gene can be found in the human genome at 6q22.33. This HINT3 gene is predicted to encode a 182 amino acid protein with a 31 and 25 amino acid extension at the N- and C-terminus, respectively. The group of Rick Wagner (University of Minnesota) reported that Hint3 can assemble into multimeric oligomers and prefers acyl-adenylates to nucleoside phosphoramidates. Hint3 seems to form aggregates both in the cytosol and in the nucleus.

2. Aprataxin

In 2001, linkage analysis showed that aprataxin was mutated in patients with ataxia-ocular apraxia. The aprataxin gene (APTX) encodes a member of the histidine triad (HIT) superfamily and plays a role in DNA single-strand-break repair. Aprataxin catalyses the nucleophilic release of adenylate groups covalently linked to 5'-phosphate termini at single-strand nicks and gaps, resulting in the production of 5'-phosphate termini that can be efficiently rejoined. Aprataxin has also been localized to mitochondria where it protects mtDNA. DNA repair functions of aprataxin have pharmacological implications in oncology: aprataxin gene expression levels were found to negatively correlate with irinotecan sensitivity in tumour samples.

3. Scavenger decapping enyzme, DCPS

The 3’-5’ mRNA degradation pathway generates small capped m7G-containing oligonucleotides (m7GpppN) that are hydrolyzed into m7GMP by the scavenger mRNA decapping enzyme DcpS, a member of the histidine triad (HIT) family of proteins.

Figure 3 - Family of human hit proteins - Reference: Hit proteins,
mitochondria and cancer. Martin J. et al., 2011 Biochim Biophys Acta



 

4. FHIT (Fragile Hit protein)

The FHIT gene is localized within the most fragile region of the human genome. FHIT is inactivated in many types of human tumours like lung, oesophagus, stomach, kidney, cervical and hepatocellular carcinomas, indicating that Fhit is a pro-apoptotic tumour suppressor. More recently Fhit was found to have not only a cytosolic but also mitochondrial localization where it stabilizes ferredoxin reductase. Fhit-negative cells produce lower levels of apoptosis-inducing reactive oxygen species.

5. Galactose-1-phosphate uridylyltransferase

Galactose-1-phosphate uridylyltransferase, which contains tandem HIT domains, is the second enzyme in the metabolism of galactose in the liver. It catalyzes the transfer of UMP from UDP-glucose to galactose-1-phosphate producing UDP-galactose and glucose-1-phosphate. Mutations in the galactose-1-phosphate uridylyltransferase gene result in galactosemia, a disease characterized by a failure to thrive, cataracts and mental retardation.