The microtubules (MTs) of most vertebrate tissue cellswill disassemble at low temperature, but some remaincold-stable or resistant to drugs such as nocodazole. It hasbeen shown that MT cold- and nocodazole-resistance islargely due to the association with the class of MicrotubuleAssociated Proteins (MAP) known as MAP6 (previouslynamed STOP for Stable Tubule Only Polypeptide) [1].MAP6 proteins are expressed only in vertebrates, andhave been localized in neurons, astrocytes, oligodendrocytes,fibroblasts, and several tissues. In eukaryotes, theMT-based organelles centrioles, cilia and flagella MT havecold-resistant MTs, but, so far, MAP6 proteins have notbeen characterized in these organelles. We have recentlyidentified TbSAXO (for Stop AXOneme), a novel flagellarprotein in the protozoan parasite Trypanosoma brucei.We show here that TbSAXO is a microtubule stabilizingprotein with properties similar, upon cold and nocodazoletreatment, to those of the microtubule-stabilizing Mndomains of the MAP6 proteins, thus identifying the firstMAP6-related protein in a protozoan. Further, we demonstrate,in the parasite, that TbSAXO is an axonemeassociatedprotein, which plays a role in flagellum motility.We also show that TbSAXO is the first member of agroup of MAP6-related proteins (that we named SAXOproteins) present only in organisms with centrioles / cilia /flagella and ranging from protozoa to mammals, suggestingpotential roles of the SAXO proteins in cilia andflagella function.
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