Hits, Fhits and Nits: beyond enzymatic function

Abstract

We have briefly summarized what is known about these proteins, but in closing wish to feature the outstanding questions. Hint1 was discovered mistakenly as an inhibitor of Protein Kinase C and designated Pkci, a designation that still confuses the literature. The other Hint family members were discovered by homology to Hint1. Aprataxin was discovered as a result of the hunt for a gene responsible for AOA1. Fhit was discovered through cloning of a familial chromosome translocation breakpoint on chromosome 3 that interrupts the large FHIT gene within an intron, in the FRA3B chromosome region (Ohta et al., 1996), now known to be the region of the human genome most susceptible to DNA damage due to replication stress (Durkin et al., 2008). The NitFhit fusion genewas discovered during searches for Fhit homologs in flies and worms because the fly/worm Nit polypeptide is fused to the 5'-end of the Fhit gene; the mammalian Nit gene family was discovered because of the NitFhit fusion gene, in searches for homologs to the Nit polypeptide of the NitFhit gene. Each of the Hit family member proteins is reported to have enzymatic activities toward putative substrates involving nucleosides or dinucleosides. Most surprisingly, each of the Hit family proteins discussed has been implicated in important DNA damage response pathways and/or tumor suppression pathways. And for each of them it has been difficult to assign definite substrates, to know if the substrates and catalytic products have biological functions, to know if that function is related to the DNA damage response and suppressor functions, and to precisely define the pathways through which tumor suppression occurs. When the fly Nit sequence was found at the 5'-end of the fly Fhit gene, this gene was hailed as a Rosetta stone gene/protein that would help in discovery of the function of Fhit, because the Nit protein should be in the same signal pathway (Pace et al., 2000). However, the mammalian Nit family proteins have turned out to be at least as mysterious as the Fhit proteins, with the Nit1 substrate still unknown and the surprising finding that Nit proteins also appear to behave as tumor suppressor proteins. Whether the predicted enzymatic functions of these proteins are relevant to the observed biological functions, remain among the outstanding unanswered puzzles and raise the question: have these mammalian proteins evolved beyond the putative original enzymatic purpose, such that the catalytic function is now vestigial and subservient to signal pathways that use the protein-substrate complexes in pathways that signal apoptosis or DNA damage response? Or can these proteins be fulfilling catalytic functions independently but in parallel with signal pathway functions, as perhaps observed for Aprataxin? Or is the catalytic function indeed part of the observed biological functions, such as apoptosis and tumor suppression? Perhaps the recent, post-genomic focus on metabolomics and genome-wide investigations of signal pathway networks will lead to answers to some of these outstanding questions.

Fig. 1. Alignment of amino acid sequences to show the relationships among Hint, Fhit and Nit proteins

Human Hint1 and 2 (red), C. elegans and D. melanogaster NitFhit (blue/black), human Fhit (black) and human Nit1 and 2 (blue) protein sequences were aligned using ClustalW. The most highly conserved amino acids are highlighted yellow (Nit specific) or green (Fhit specific). The signature, conserved Cys and His are starred for Nits and Fhits/Hints, respectively.

Fig. 2. Subcellular localization of Fhit in MCF7 breast cancer cells

After isolation of nuclear and cytoplasmic cellular fractions from MCF7 breast cancer cells expressing abundant Fhit,50 µg protein was loaded to each lane of the gel, proteins were transferred and assessed by immunoblot for expression of Fhit in specific subcellular fractions. GAPDH was used as a cytosolic marker and PARP as nuclear marker.