finding that it stimulates the formation of the dipeptide analogue methionyl-puromycin in an in vitro assay that mimics the formation of the first peptide bond in protein synthesis

Human eukaryotic initiation factor 5A (eIF-5A) is a small acidic protein of 154 amino acids (aa) with a molecular mass of 16.7 kDa (Smit-McBride et al., 1989a). eIF-5A is unique, because it is the only cellular protein known to date to contain the unusal amino acid hypusine (reviewed by Park et al., 1993a). Although hypusine-modified eIF-5A appears to be essential for eukaryotic cell proliferation (reviewed by Park et al., 1993b), its precise function still remains elusive. Its designation as an ‘initiation factor’ originated from the initial finding that it stimulates the formation of the dipeptide analogue methionyl-puromycin in an in vitro assay that mimics the formation of the first peptide bond in protein synthesis (Park, 1989; Smit-McBride et al., 1989b). The activity of eIF5A in this model assay, however, appears to be poor (Kang et al., 1993). More recent in vivo data has clearly indicated that eIF-5A is not an initiator of general protein translation. In fact, complete intracellular depletion of eIF-5A in the yeast Saccharomyces cerevisiae does not cause major changes in the overall rate of protein synthesis (Kang and Hershey, 1994). Thus, the precise in vivo function of eIF-5A in eukaryotic cells is as yet unknown. Investigation of the posttranscriptional regulation of human immunodeficiency virus type 1 (HIV-1) gene expression indicated a potentially novel eIF-5A activity in vivo. HIV-1 requires the action of the viral Rev trans-activator protein in order to produce progeny viruses (for a detailed review see Pollard and Malim, 1998). Rev constantly shuttles between the nucleus and cytoplasm of host cells and mediates the translocation of unspliced and incompletely-spliced viral mRNAs across the nuclear envelope. In particular, nuclear export of Rev is mediated by a distinct effector or activation domain, which is composed of four critically spaced leucine residues and also functions as a nuclear export signal (NES). Indeed, various proteins have been described that bind specifically to this domain and may therefore mediate Rev function. This includes the human nucleoporin-like protein hRIP/Rab (Bogerd et al., 1995; Fritz et al., 1995), the export factor CRM1 (Stade et al., 1997; Ossareh-Nazari et al., 1997; Fornerod et al., 1997a; Fukuda et al., 1997) and the hypusinecontaining protein eIF-5A (Ruhl et al., 1993; Bevec et al., 1996). In the case of eIF-5A in particular, it has been possible to describe distinct eIF-5A mutant proteins that upon intracellular expression inhibit Rev trans-activation and thereby HIV-1 replication in human T-cells (Bevec et al., 1996; Junker et al., 1996). Furthermore, microinjection studies demonstrated that these eIF-5A mutant proteins or antibodies directed against eIF-5A block the nuclear export of Rev in somatic cells (Bevec et al., 1996; Schatz et al., 1998); this suggests that eIF-5A is part of a nucleocytoplasmic transport pathway. In eukaryotic cells, the nuclear envelope creates two distinct cellular compartments that separate DNA replication and transcription from protein synthesis. Thus, a constant flow of macromolecules has to traverse the nuclear envelope in both 2369 Journal of Cell Science 112, 2369-2380 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS0251