To understand how signaling proteins function, it is crucial to know the timeordered sequence of events that lead to the signaling state. When the messenger is chemical, the time required to diffuse to and bind in the active site of a signaling protein is typically far longer than the timescale for protein conformational change [1]. For the structural determination of the kinetics of enzymatic reactions we will focus on small GTPases and their co-enzymes. Small GTPases are molecular switches that cycle between a GTP-bound active and a GDP-bound inactive form. The switch is catalyzed by Guanine nucleotide Exchange Factors (GEFs) and GTPase-Activating Proteins (GAPs), the latter catalyze the hydrolysis of GTP to GDP to deactivate the small GTPase. This system is of very high, general importance in cell biology with particular impact on disease processes, especially cancer, but also several infectious diseases. For proof-ofprinciple, we chose the Arl3-RP2 complex as GTPase-GAP pair [2]. The gene encoding for the GAP protein RP2 (Retinitis pigmentosa 2) is highly mutated in patients of X-linked Retinitis pigmentosa, with mutational hotspots in residues catalyzing the GAP reaction on Arl3. Retinitis pigmentosa refers to a heterogeneous group of inherited ocular diseases that result in a progressive retinal degeneration affecting 1 in 3,000?5,000 people.

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