Acylation-dependent protein export in Leishmania

The surface of the protozoan parasite Leishmania is unusual in that it cons ists predominantly of glycosylphosphatidylinositol-anchored glycoconjugates and proteins. Additionally, a family of hydrophilic acylated surface prote ins (HASPs) has been localized to the extracellular face of the plasma memb rane in infective parasite stages. These surface polypeptides lack a recogn izable endoplasmic reticulum secretory signal sequence, transmembrane spann ing domain, or glycosylphosphatidylinositol-anchor consensus sequence, indi cating that novel mechanisms are involved in their transport and localizati on, Here, we show that the N-terminal domain of HASPB contains primary stru ctural information that directs both N-myristoylation and palmitoylation an d is essential for correct localization of the protein to the plasma membra ne, Furthermore, the N-terminal 18 amino acids of HASPB, encoding the dual acylation site, are sufficient to target the heterologous Aequorea victoria green fluorescent protein to the cell surface of Leishmania. Mutagenesis o f the predicted acylated residues confirms that modification by both myrist ate and palmitate is required for correct trafficking. These data suggest t hat HASPB is a representative of a novel class of proteins whose translocat ion onto the surface of eukaryotic cells is dependent upon a "nonclassical" pathway involving N-myristoylation/palmitoylation. Significantly, HASPB is also translocated on to the extracellular face of the plasma membrane of t ransfected mammalian cells, indicating that the export signal for HASPB is recognized by a higher eukaryotic export mechanism.