SMP-1, a member of a new family of small myristoylated proteins in kinetoplastid parasites, is targeted to the flagellum membrane in Leishmania

Abstract

The mechanisms by which proteins are targeted to the membrane of eukaryotic flagella and cilia are largely uncharacterized. We have identified a new family of small myristoylated proteins (SMPs) that are present in Leishmania spp and related trypanosomatid parasites. One of these proteins, termed SMP-1, is targeted to the Leishmania flagellum. SMP-1 is myristoylated and palmitoylated in vivo, and mutation of Gly-2 and Cys-3 residues showed that both fatty acids are required for flagellar localization. SMP-1 is associated with detergent-resistant membranes based on its recovery in the buoyant fraction after Triton X-100 extraction and sucrose density centrifugation and coextraction with the major surface glycolipids in Triton X-114. However, the flagellar localization of SMP-1 was not affected when sterol biosynthesis and the properties of detergent-resistant membranes were perturbed with ketoconazole. Remarkably, treatment of Leishmania with ketoconazole and myriocin (an inhibitor of sphingolipid biosynthesis) also had no affect on SMP-1 localization, despite causing the massive distension of the flagellum membrane and the partial or complete loss of internal axoneme and paraflagellar rod structures, respectively. These data suggest that flagellar membrane targeting of SMP-1 is not dependent on axonemal structures and that alterations in flagellar membrane lipid composition disrupt axoneme extension.