# Roco Proteins: GTPases with a Baroque Structure and Mechanism
## 5 The Central RocCOR Domain Tandem of Roco Proteins: Structural Insights
This is in strong contrast with the region around the switch II DXXG motif (G3), which is the most highly conserved region of Roco proteins, together with the residues on the interface between the Roc and COR domain. The switch II loop of Roc is resolved in the structure and makes contact with the most highly conserved patch of the COR domain of the same protomer. Figure 2: Crystal structure of the swapped dimer of the LRRK2 Roc domain bound to GDP (pdb: 2 zej) [104]. The two swapped G-domains are shown in orange and pink, respectively. For one active site the P-loop or G1 motif (cyan), the switch I or G2 motif (light pink), the switch II or G3 motif (yellow), the G4 (red), and G5 motif (blue) are highlighted. Each active site is composed of the G1–3 motif of one protomer and the G4–5 motif of the other. GDP is depicted as a green stick model an Mg\({}^{2+}\) as a yellow sphere. Consistent with the observed implication of the C-terminal half of the COR domain of _Ct_Roco in dimer formation, analytical gel filtration showed that the RoCOR construct elutes at a volume corresponding to a dimer. However, a RoCOR\(\Delta\)C protein construct, which lacks the C-terminal half of the COR domain, elutes at a volume corresponding to a monomer confirming an important role of this part of the protein in dimerization [33]. In 2015, [PERSON] et al. reported the crystal structure of a monomeric RoCOR\(\Delta\)C construct, lacking the C-terminal half of the COR domain, from the Roco2 protein from the prokaryotic archaeon _Methanosarcina barkeri_ (_Mb_) in the GDP-bound state. Comparison of this structure with nucleotide-free _Ct_RocoCOR revealed a rearrangement of the switch II region  
Figure 3: The C-terminal half of the COR domain is important for dimerization (pdb: 3 DPU, 4 WNR and 6 HLU). **(a)** The crystal structure of the RoCOR\(\Delta\)C domain construct from the Roco2 protein of _Methanosarcina barkeri_ reveals a monomeric conformation [33]. **(b)** The crystal structure of the RoCOR domain construct from the Roco protein of _Chlorobium__teplatum_ shows dimer formation via the C-terminal COR domains. The COR domain contains two subdomains: an N-terminal, mainly \(\alpha\)-helical domain with a short antiparallel \(\beta\)-sheet (N-COR), and a C-terminal domain with a \(\beta\)-sheet surrounded by \(\alpha\)-helices and a \(\beta\)-hairpin involved in dimerization (C-COR). Only one Roc domain is resolved in the structure, presumably due to the flexibility of the other Roc domain. The putative site of the second Roc domain is indicated with a black dotted line [33]. **(c)** The crystal structure of the LRR-RocCOR domain construct from _Ct_Roco. This structure reveals the second Roc domain and the orientation of the LRR domains with respect to the other domain. Apart from the COR-COR interactions, also Roc–Roc and Roc–COR interactions contribute significantly to the dimer interface [17]. causing a shift of the N-terminal COR domain. Since it is proposed that G-proteins are molecular switches that cycle between a GTP- and GDP-bound state, and couple this effect to a downstream effector, it might be that the switch II region of the Roc domain plays a role in this intramolecular signaling [34]. Using symmetry arguments, the second Roc domain was modeled in the C_t_RocCOR structure, resulting in a model where the two Roc domains face each other with their nucleotide-binding sites in a head-to-tail fashion. This observation, thus, suggested that Roco proteins would form constitutive dimers predominantly via their COR domains, while the Roc domains are facing each other. Modeling of the non-hydrolysable GTP-analogue GppNHp into the nucleotide binding site, moreover, showed that the ribose of one nucleotide would face the \(\gamma\)-phosphate of the adjacent nucleotide and vice versa. A further zoom in on the nucleotide binding site revealed the presence of an arginine residue in the Roc domain that could contact the nucleotide binding site of the other Roc domain within the dimer.