A nuclear
protein in Schizosaccharomyces pombe with homology to the human tumour
suppressor Fhit has decapping activity.
Zivar Salehi (1),
Lars Geffers (2), Cristina Vilela (2), Marina Ptushkina (2), Ralf Birkenhager
(2), Karine Bertelot (2), Ian Hagan (3), John McCarthy (2)
(1) Biology, The University of Guilan, Namjo, Rasht, 41335, Iran
(geneticzs@yahoo.co.uk); (2) Department of Biomolecular Sciences Department of
BMS UMIST, PO Box 88, Manchester M60 1QD, UK; (3) Paterson Institute for Cancer
Research, Wilmslow Road, Manchester M20 4BX, UK
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 m7GTP-Sepharose, albeit with lower specificity and affinity for methylated GTP than is typical for the cap-binding 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.