Characterization of a differentially expressed protein that shows an unusual localization to intracellular membranes in Leishmania major

The SHERP genes are found as a tandem pair within the differentially regula ted LmcDNA16 locus of Leishmania major. The SHERP gene product ((s) under b ar mall (h) under bar ydrophilic (e) under bar ndoplasmic (r) under bar eti culum-associated (p) under bar rotein) is unusual in its small size (6.2 kD a), its acidic pI (4.6) and its exclusive, high-level expression (approxima te to 100000 copies per cell) in infective non-replicative parasite stages. No homologues have been found to date. Secondary-structure predictions sug gest that SHERP contains an amphiphilic alpha -helix that is presumably inv olved in protein-protein interactions. SHERP has been localized to the endo plasmic reticulum as well as to the outer mitochondrial membrane in both wi ld-type and over-expressing parasites. Given the absence of an N-terminal s ignal sequence, transmembrane- spanning domains or detectable post-translat ional modifications, it is likely that this hydrophilic molecule is a perip heral membrane protein on the cytosolic face of intracellular membranes. Th is weak membrane association has been confirmed in cell-fractionation assay s, in which SHERP redistributes from the cytoplasmic to the membrane fracti on after in vivo cross-linking. SHERP does not appear to be involved in rea rrangements of the cytoskeleton or conservation of organelle morphology dur ing parasite differentiation. The role of this novel protein, presumed to b e part of a protein complex, in infective parasites that are nutrient-defic ient and pre-adapted for intracellular survival in the mammalian host is un der investigation.