Rab2B function in slender-to-stumpy differentiation.

The Rab family of small GTPases are regulators of intracellular membrane trafficking. Rab2B, a Golgi-resident Rab GTPase, has been shown to function in ER-Golgi transport and autophagosome-lysosome fusion in mammalian cells and Drosophila. In T. brucei, the autophagy-related function of Rab2B was conserved and depletion of Rab2B blocked autophagosome-lysosome fusion. Intriguingly, depletion of Rab2B induced differentiation of T. brucei from a proliferative slender form to a quiescent stumpy form, a step crucial for parasite transmission. Chaperonin CCT/TRiC was identified as an effector of Rab2B. Depletion of CCT subunit or actin (a known CCT/TRiC substrate), or blocking actin polymerisation with LatA all triggered differentiation, suggesting a role of Rab2B on CCT/TRiC-mediated actin folding. Using an actin chromobody, we identified a pool of nuclear actin that is crucial for transcription reprogramming and life cycle differentiation in T. brucei. The study revealed a previously unknown function of Rab2B and actin in T. brucei differentiation, and an unexpected link between Rab2B and CCT/TRiC function.