The phylogenetically conserved eukaryotic translation initi- ation factor 5A(eIF5A) is the onlyknowncellular protein to contain the post-translationally derived aminoacid hypusine [Ne-(4-amino-2-hydroxybutyl)lysine]. Both eIF5A and its hypusine modification are essential for sustained cell proli- feration. Normally only one eIF5A protein is expressed in human cells.Recently, we identified a second human EIF5A gene that would encode an isoform (eIF5A-2) of 84% sequence identity. Overexpression of eIF5A-2 mRNA in certain human cancer cells, in contrast to weak normal expression limited to human testis and brain, suggests EIF5A2 as a potential oncogene. However, eIF5A-2 protein has not been described in human or mammalian cells here- tofore. Here, we describe the identification of eIF5A-2 pro- tein in human colorectal and ovarian cancer lines, SW-480 and UACC-1598, that overexpress eIF5A-2 mRNAs. Functional characterization of the human isoforms revealed that either human EIF5A gene can complement growth of a yeast strain in which the yeast EIF5A genes were disrupted. This indicates functional similarity of the human isoforms in yeast and suggests that eIF5A-2 has an important role in eukaryotic cell survival similar to that of the ubiquitous eIF5A-1. Detectable structural differences were also noted, including lack of immunological cross-reactivity, formation of different complexes with deoxyhypusine synthase, and Km values (1.5 ± 0.2 vs. 8.3 ± 1.4 lM for eIF5A-1 and -2, respectively) as substrates for deoxyhypusine synthase in vitro. These physical characteristics and distinct amino acid sequences in the C-terminal domain together with differences in gene expression patterns imply differentiated, tissue-specific functions of the eIF5A-2 isoform in the mammalian organism and in cancer.
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