A nuclear protein in Schizosaccharomyces pombe with homology to the human tumour suppressor Fhit has decapping activity

Abstract

SummaryA number of eukaryotic proteins are already known to orchestrate key steps of mRNA metabolism and translation via interactions with the 5¢ ¢ ¢ ¢ m 7 GpppN cap. We have characterized a new type of histidine triad (HIT) motif protein (Nhm1) that co-purifies with the cap-binding complex eIF4F of Schizosaccharomyces pombe . Nhm1 is an RNA-binding protein that binds to m 7 GTP-Sepharose, albeit with lower specificity and affinity for methylated GTP than is typical for the capbinding protein known as eukaryotic initiation factor 4E. Sequence searches have revealed that proteins with strong sequence similarity over all regions of the new protein exist in a wide range of eukaryotes, yet none has been characterized up to now. However, other proteins that share specific motifs with Nhm1 include the human Fhit tumour suppressor protein and the diadenosine 5¢ ¢ ¢ ¢ , 5¢ ¢ ¢ ¢¢ ¢ ¢ ¢¢ ¢ ¢ ¢ -P1, P4-tetraphosphate asymmetrical hydrolase of S. pombe . Our experimental work also reveals that Nhm1 inhibits translation in a cell-free extract prepared from S. pombe , and that it is therefore a putative translational modulator. On the other hand, purified Nhm1 manifests mRNA decapping activity, yet is physically distinct from the Saccharomyces cerevisiae decapping enzyme Dcp1. Moreover, fluorescence and immunofluorescence microscopy show that Nhm1 is predominantly, although not exclusively, nuclear. We conclude that Nhm1 has evolved as a special branch of the HIT motif superfamily that has the potential to influence both the metabolism and the translation of mRNA, and that its presence in S. pombe suggests the utilization of a novel decapping pathway.

Functional characterization of the mammalian mRNA decapping enzyme hDcp2

“…The DcpS homolog, Dcs1p, carries out the same function in Saccharomyces cerevisiae (Liu et al 2002). Interestingly, a similar protein was reported recently in Schizosaccharomyces pombe, and it hydrolyzes the cap of an intact mRNA substrate (Salehi et al 2002). …”

31P NMR and Genetic Analysis Establish hinT as the Only Escherchia coli Purine Nucleoside Phosphoramidase and as Essential for Growth under High Salt Conditions

“…The hint1 homolog of mouse is a nonessential gene (7), the loss of which predisposes to an increased incidence of tumors (8). Two less typical members of the Hint branch of the superfamily are Aprataxin, which is mutated in ataxia-oculomotor apraxia1 (9,10), and the scavenger-decapping enzyme, Dcps (11)(12)(13). The second and third branches of the HIT superfamily are homologs of the Fhit tumor suppressor protein and enzymes related to galactose-1-phosphate uridylyltransferase (1).…”

Identification of the HIT-45 protein from Trypanosoma brucei as an FHIT protein/dinucleoside triphosphatase: Substrate specificity studies on the recombinant and endogenous proteins

“…Representative proteins within each group are limited to those for which detailed functional studies have been published. References: Human FHIT (Brenner 2002); S. pombe Aph1p (Ingram and Barnes 2000), A. thaliana FHIT (Guranowski et al 2008), trypanosome HIT-45 (this paper), human DcpS (Gu et al 2004), S. cerevisiae Dcs1p2p (Liu et al 2002), C. elegans dcs-1 (Cohen et al 2004), S. pombe Nhm1p (Salehi et al 2002), human Hint1 (Brenner et al 1997), S. cerevisiae Hnt1p and Gal7p (Tajima et al 1985).…”