A rab-related GTP-binding protein in Schistosoma mansoni

Schistosomiasis is a parasitic disease initiated by the deposition of eggs in host tissues by the blood fluke Schistosoma. A gene encoding a low-molecular weight GTP-binding protein (LMWGP) was cloned from Schistosoma mansoni using a polymerase chain reaction (PCR)-based strategy. The gene was termed smrab (Schistosoma mansoni rab-related protein). Northern blot analysis hybridizes smrab cDNA with a 1.5-kb band of mRNA; this mRNA is abundantly expressed in male schistosomes, while only slightly in females. The deduced amino acid sequence of smrab shares ca. 70% homology with that of several rab-related LMWGPs. Smrab terminates in a CCXXX motif, which is one of several signals for post-translational isoprenoid modification by geranylgeranyl protein transferase (GGPT) type II. A GGPT assay with in vitro translation product confirms that smrab is geranylgeranylated. Recombinant expression of smrab in the pET3a expression vector yields insoluble protein which migrates as a 23-kDa band on SDS-PAGE. N-terminal sequence information of the recombinant protein matches the predicted amino acid sequence of smrab. GTP-binding analysis indicates that the recombinant protein binds GTP. Therefore, smrab meets the criteria recently established for acceptance into the ras superfamily of GTP-binding proteins (Kahn, R.A., Der, C.J. and Bokoch, G.M. (1992) The ras superfamily of GTP-binding proteins: guidelines on nomenclature. FASEB J. 6, 2512-2513, Ref. [21]).