# Roco Proteins: GTPases with a Baroque Structure and Mechanism
## 4 Physiological Functions of Roco Proteins
### Roco Proteins from Vertebrates  
Four Roco proteins have been discovered in vertebrates so far, which in humans are called malignant fibrous histiocytoma amplified sequence 1 (MFHAS1), death-associated protein kinase 1 (DAPK1), and leucine-rich repeat kinase 1 (LRRK1) and 2 (LRRK2) [28]. MFHAS1 belongs to the first group of Roco proteins and only consists of an N-terminal LRR domain followed by a central RocCOR domain tandem. On top of the enhanced expression of MFHAS1 in malignant fibrous histiocytomas [42], the gene coding for MFHAS1 was found to be a potential oncogene for gastric, oesophageal, and gastroesophageal cancers [43; 44; 45]. Despite its oncogenic properties, MFHAS1 is only poorly characterized. A first biochemical study by [PERSON] et al. showed that it is a guanosine-5'-triphosphate (GTP)-binding protein with very low endogenous GTPase activity and that the oligomeric state of the protein is influenced by nucleotide binding. However, the precise nature of the observed oligomeric complexes remains to be determined [46]. DAPK1 is part of the third group of Roco proteins, containing a calmodulin-dependent serine/threonine kinase domain, and binding to the cytoskeleton. It is an essential regulator of various cell death signaling pathways and is also important for the activation of autophagy [27; 47; 48; 49]. It has been linked to several neuronal pathologies, including Alzheimer's disease and ischemia-induced neuronal cell death [50]. DAPK1 also suppresses the cellular transformation during the early stages of tumor development [27; 47; 51; 52]. Deletion or downregulation of DAPK1 causes chronic lymphocytic leukemia [27; 53]. The protein forms a dimer through its Roc domain and its calmodulin (CaM)-dependent kinase domain [54; 55; 56]. Intriguingly, binding of GTP to the Roc domain was reported to have an inhibitory effect on the kinase activity of DAPK1 [54; 55]. Upon binding of GTP to the Roc domain, the kinase domain undergoes a conformational change that leads to autophosphorylation of a serine residue in its CaM-regulatory domain. This autophosphorylation decreases the protein's catalytic activity. Upon GTP hydrolysis, DAPK1 kinase activity is reactivated [50; 52; 54]. LRRK1 and LRRK2 both belong to the second group of the Roco proteins and have a similar domain topology consisting of N-terminal anlyrin and leucine-rich repeats, followed by a central core of a Roc, COR and kinase domain and finally a C-terminal WD40 domain. LRRK2 has an additional N-terminal armadillo repeat domain preceding the anlyrin repeats [29]. Although LRRK1 and LRRK2 share a similar domain topology, both proteins clearly exhibit different cellular functions [15; 27; 57; 58]. LRRK1 is required for B-cell receptor-mediated B-cell proliferation and survival, and in this way, plays a crucial role in humoral immunity [28; 59]. It also has a regulatory function in mitotic spindle orientation. Polo-like kinase 1 phosphorylates LRRK1 on position Ser1790. This activated LRRK1 subsequently phosphorylates the CDK5 regulatory subunit associated protein 2 (CDK5 RAP2), which is responsible for centrosome maturation [28; 60]. In addition LRRK1 plays a crucial role in epidermal growth factor receptor (EGFR) sorting and transport [28; 57; 61; 62]. The endosomal trafficking of EGFR regulates the amplitude and duration of EGFR signaling [61; 62]. Dysregulation of this signaling has been linked to several types of human cancers [62; 63]. Activated EGFR is internalized via endocytosis, resulting in the phosphorylation and activation of LRRK1, which in turn regulates the motility of the EGFR-containing endosome. Non-phosphorylated LRRK1 mutants display increased endosome motility and accumulation of EGFR in mixed endosomes [57; 62]. On the other hand, LRRK1 also playsa role in the endosomal sorting process by binding EGFR through its adaptor protein growth factor receptor-bound protein 2 (Grb2) and associating it with the ESCRT-0 sorting complex [57, 61].