Crystal structure of a Chlamydomonas reinhardtii flagellar RabGAP TBC-domain at 1.8 Å resolution.

Rab GTPases play a crucial role in the regulation of many intracellular membrane trafficking pathways including endocytosis and ciliogenesis. Rab GTPase activating proteins (RabGAPs) increase the GTP hydrolysis rate of Rab GTPases and turn them into guanine nucleotide diphosphate (GDP) bound inactive form. Here, we determined the crystal structure of the putative catalytic domain of a RabGAP (which we name CrfRabGAP) that is found in the flagellar proteome of the unicellular green alga Chlamydomonas reinhardtii. BLAST searches revealed potential human orthologues of CrfRabGAP as TBC1D3 and TBC1D26. Sequence and structural comparison with other canonical RabGAPs revealed that the CrfRabGAP does not contain the canonical catalytic residues required for the activation of Rab GTPases. The function of noncanonical RabGAPs-like CrfRabGAP might be to serve as Rab effectors rather than activators.