# Roco Proteins: GTPases with a Baroque Structure and Mechanism
## 8 Perspectives  
The above-described model clearly generates new insights, but also raises many new important questions. Although it has been shown that CfRoco switches between a monomeric and dimeric state during GTP hydrolysis, the structural mechanism and functional implications for this transition remains to be determined [19]. Interestingly, in contrast to many other small Ras-like GTPases which display very high affinity for GTP and GDP, our recent data suggest that the GTPase activity of Roco proteins might be directly dependent on the physiological levels of GTP [18]. The conserved metabolite GTP drives chemical reactions in the cell. However, the cellular status strongly influences the concentration of GTP, which varies between 0.1 and 1 mM [123; 124]. The Michaelis-Menten constant (K\({}_{\rm M}\)) of the GTPase reaction of full length Roco proteins and LRRK2 lies in a similar range as the cellular concentration of GTP, suggesting that the rate of GTP hydrolysis will scale with the GTP concentration and that Roco proteins might, thus, act as GTP sensors rather than as classical switches. However, future research is clearly needed to further explore this potential function of Roco proteins [100; 126]. Moreover, it is as yet unclear whether GTP hydrolysis and protein re-dimerization coincide or are actually coupled to each other. Furthermore, although PD analogous mutations have been shown to affect the Roco monomer/dimer equilibrium, further research is necessary to confirm that a similar mechanism actually holds true for LRRK2. The PD mutations within the LRRK2 Roc (R1441C/G/H) and COR (Y1699C) all have reduced GTPase activity. However, the exact effect of these PD mutations  
Figure 5: A newly proposed working mechanism for LRRK2. Cytosolic GTP-bound monomeric LRRK2 is recruited to the cell membrane by binding via its N-terminal domains to GTP-bound Rab proteins that are located at the membrane. At the cell membrane, GTP is hydrolys, and the protein dimerizes. Meanwhile, the LRRK2 kinase domain is activated, and Rab proteins are phosphorylated. The low affinity of LRRK2 for GDP then leads to fast GDP release. Due to the higher GTP concentration present in the cell, this results in rebinding of GTP, monomerization of LRRK2 and return to the cytosol.  
on dimerization remains to be determined. Interestingly, modelling of the LRRK2 pathogenic mutations on our recently _Cf_LRR-Roc-COR structure revealed that these mutations are located within the same regions that are important for nucleotide-dependent monomer-dimerization [17]. According to our model, Roco proteins and presumably LRRK2 need to cycle through a monomeric and dimeric state, thus, stabilization by PD-mutations of either state could lead to the observed reduced GTPase activity of the PD mutants. The link between the imbalance in homodimerization and PD, thus, creates an appealing drug target [19]. Besides the RocCOR domain tandem, LRRK2 also contains a kinase domain and several protein-protein interaction domains [133]. Both the GTPase and the kinase function of LRRK2 are essential, and depend on each other, for proper functioning [12; 128]. Further research is needed to unravel how these two domains interact and how additional regulatory factors like phosphorylation, binding to effectors and cellular localization influence the protein's functioning and oligomerization [107; 136; 140; 141].  
[PERSON], [PERSON] and A.K. wrote and edited the manuscript.  
Work in the author's laboratories is supported by a VUB/RUG collaboration agreement (OZR2544; L.W.), the Fonds voor Wetenschappelijk Onderzoek (W.V.), a Strategic Research Program Financing of the VUB (W.V.), The Michael J. Fox Foundation for Parkinson's Research (A.K., W.V.) and an NWO-VIDI grant (A.K.).  
The authors declare no conflict of interest.