Functional significance of eIF5A and its hypusine modification in eukaryotes

Abstract

The unusual basic amino acid, hypusine [N(epsilon)-(4-amino-2-hydroxybutyl)-lysine], is a modified lysine with the addition of the 4-aminobutyl moiety from the polyamine spermidine. This naturally occurring amino acid is a product of a unique posttranslational modification that occurs in only one cellular protein, eukaryotic translation initiation factor 5A (eIF5A, eIF-5A). Hypusine is synthesized exclusively in this protein by two sequential enzymatic steps involving deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). The deoxyhypusine/hypusine synthetic pathway has evolved in archaea and eukaryotes, and eIF5A, DHS and DOHH are highly conserved suggesting a vital cellular function of eIF5A. Gene disruption and mutation studies in yeast and higher eukaryotes have provided valuable information on the essential nature of eIF5A and the deoxyhypusine/hypusine modification in cell growth and in protein synthesis. In view of the extraordinary specificity and functional significance of hypusine-containing eIF5A in mammalian cell proliferation, eIF5A and the hypusine biosynthetic enzymes are novel potential targets for intervention in aberrant cell proliferation.

Fig. 1

Hypusine biosynthesis in eIF5A. The polyamine spermidine, which is synthesized from putrescine, is the source of the aminobutyl moiety of hypusine, as indicated by shading. Hypusine synthesis occurs at one specific lysine residue of the eIF5A precursor protein, eIF5A(Lys), by two enzymatic steps, involving deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). Modified from Park 2006

Fig. 2

Conservation of the amino acid sequence of eIF5A in eukaryotes (a), crystal structure of human eIF5A(Lys) (b) and comparison of crystal structures of aIF5A with EF-P(c). a The numbers on top of the bar indicate the amino acid residue number for human eIF5A. One lysine (Lys50) is converted to hypusine residue and the sequence surrounding this modification site is highly conserved. b crystal structure of truncated human eIF5A(Lys) (aa15–151) protein (PDB:3cpf; Tong et al. 2009), eIF5A consists of two domains, N- and C-terminal domains of β-sheet structure. The hypusine modification site (K50) is located at the tip of an exposed loop in the N-terminal domain. c comparison of bacterial EF-P and aIF5A structure, Thermus thermophilus EF-P consists of three domains (I, II, and III). The aIF5A structure (yellow) is superimposable on the first two domains of EF-P (blue). Modified from (Park 2006)

Fig. 3

Evolution of eIF5A and its hypusine modification pathway. eIF5A orthologs are found in eubacteria and archaea and are essential genes in each organism. The DHS gene exists in archaea, and in all eukaryotes, but not in eubacteria. DOHH gene is found only in eukaryotes. E indicates essential gene, and NE indicates non-essential gene

Fig. 4

Polysome profiles of S. cerevisiae strains harboring wild type, eIF4E, eEF2, and eIF5A mutants. Ratios between polysomes and 80S monosome peaks (P/80S) were obtained by comparing the respective areas under the graphs. Modified from Gregio et al. 2009