STRUCTURE OF A FILAMENTOUS PHOSPHOGLYCOPROTEIN POLYMER - THE SECRETEDACID-PHOSPHATASE OF LEISHMANIA-MEXICANA

The insect stage of the protozoan parasite Leishmania mexicana secrete s a filamentous acid phosphatase (secreted acid phosphatase, SAP), a p olymeric phosphoglycoprotein. The wild-type (wt) SAP filament is a cop olymer composed of two related gene products SAP1 and SAP2 which are i dentical in the enzymatically active NH2-terminal domain and the COOH- terminal domain, but differ in the length of a highly glycosylated Ser /Thr-rich repeat region (32 amino acids and 383 amino acids, respectiv ely) which is located between these domains. When expressed separately , full length SAP1, SAP2, or the NH2-terminal domain alone, are able t o assemble into filaments. The Ser/Thr-rich region is the exclusive ta rget for a novel type of O-glycosylation via phosphoserines. By using glycerol spraying/low-angle rotary metal shadowing and labelling with monoclonal antibodies it is demonstrated that the repetitive region ad opts an extended conformation forming side arms which project radially from the filament core and terminate with the COOH-terminal domain. T he length of the side arms of SAP1 and SAP2 (20 nm and 90 nm, respecti vely) corresponds to the predicted length of the Ser/Thr-rich repeat r egion of SAP1 and SAP2. Mass determination by scanning electron micros copy (STEM) shows that one morphologically, defined globular particle of the filament core is a polypeptide dimer. We propose a model for th e filament core, in which the globular NH2-terminal SAP domains form o ne strand composed of polypeptide dimers or two tightly associated str ands of monomers which may twist into a double helix, similar to actin filaments. The highly O-glycosylated side arms project from the filam ent core conferring an overall bottle-brush-like appearance. The L. me xicana SAP is compared to SAPs secreted by the closely related species L. amazonensis and L. donovani. (C) 1998 Academic Press Limited.