The actomyosin system of bloodstream form Trypanosoma brucei

Abstract text

The eukaryotic cytoskeleton consists of three main components: microtubules, actin filaments, and intermediate filaments. African trypanosomes (Trypanosoma brucei) have heavily invested in their microtubule cytoskeleton, but nonetheless retained a rudimentary actomyosin system consisting of one actin gene and two myosin genes. The exact functions of this highly reduced actomyosin system in T. brucei are not well understood. One of the myosins belongs to the ubiquitous class I family; the other to the trypanosomatid-specific class XXI family. The localisation of the class I myosin in the bloodstream form of T. brucei was refined using a combination of expansion microscopy, biochemical fractionation, and immuno-electron microscopy. Unexpectedly, around half of the myosin was cytosolic, while the remainder was strongly associated with the endolysosomal system but apparently not with the secretory pathway. The class XXI myosin was found to be expressed at a very low level, and appears to be dispensable in bloodstream form cells. Use of an anti-actin chromobody enabled the simultaneous detection of actin and myosin I for the first time in trypanosomes. Unexpectedly, there was little sign of any localisation to the flagellar pocket, suggesting a limited contribution to endocytosis. Overall, the results suggest that the actomyosin system is intimately involved in post-endocytic membrane trafficking events in T. brucei.