Recombinant rat Rasgrf1, expressed in human embryonic kidney cell line 293T and bound to endogenous human calcium-activated calmodulin (CALM1), stimulates GDP to GTP exchange on recombinant human HRAS (Farnsworth et al. 1995).
After carboxymethylation, HRAS, NRAS and KRAS4A are palmitoylated on cysteine residues upstream of the CaaX motif (residue C179 in KRAS4A, C181 in NRAS and C181 and C184 in HRAS). KRAS4B lacks upstream cysteine residues and does not undergo palmitoylation (Hancock et al, 1989; Swarthout et al, 2005; reviewed in Gysin et al, 2011; Ahearn et al, 2018). Palmitoylation is catalyzed by the DHHC9:GOLGA7 complex at the Golgi membrane (Swarthout et al, 2008).
The H-RAS-like suppressor (HRASLS) subfamily consists of five enzymes (1–5) in humans that share sequence homology with lecithin:retinol acyltransferase (LRAT). All HRASLS members possess in vitro phospholipid metabolizing abilities including phospholipase A1/2 (PLA1/2) activities and O-acyltransferase activities for the remodeling of glycerophospholipid acyl chains (Golczak et al. 2012), as well as N-acyltransferase activities for the production of N-acylphosphatidylethanolamines (Mardian et al. 2015). Acyl chain remodelling can play a key role in regulating triglyceride accumulation and energy expenditure in adipocytes, making this process a potential target for treatment of metabolic disorders causing obesity. The example here describes the N-acyltransferase activity of HRASLSs for the production of N-acylphosphatidylethanolamines (NAPEs) (Uyama et al. 2012).
Ca2+/calmodulin-binding to KRAS4B dissociates it from the plasma membrane independent of nucleotide state (Firaz et al, 2005; Sidhu et al, 2003; Sperlich et al, 2016; reviewed in Shimashu et al, 2017).
KRAS4B, unique among RAS isoforms, has been shown to bind to calmodulin (Villalonga et al, 2001; Lopez-Alcala et al, 2008). This interaction is thought to decrease the affinity of KRAS4B for the plasma membrane (Fivaz and Meyer, 2005; Sidhu et al, 2003; reviewed in Ahearn et al, 2018; Nussinov et al, 2015). Interaction between oncogenic KRAS4B and calmodulin has been shown to promote tumorigenesis by interfering with the activation of CAMK2. This in turn relieves the suppression of beta-catenin dependent signaling mediated by the non-canonical WNT signaling pathway (Wang et al, 2015). The interaction between KRAS4B and calmodulin is inhibited by PKC- or PRKG2-dependent KRAS4B phosphorylation at serine 181 (Wang et al, 2015; Alvarez-Moya et al, 2010; reviewed in Ahearn et al, 2018).
Members of the RAS gene family were the first oncogenes to be identified, and mutations in RAS are present in ~20-30% of human cancers (reviewed in Prior et al, 2012). Mutations in the KRAS gene are the most prevalent, and are found with high frequency in colorectal cancer, non-small cell lung cancer and pancreatic cancer, among others. The reasons for the lower prevalence of HRAS and NRAS mutations in human cancers are not fully understood, but may reflect gene-specific functions as well as differential codon usage and spatio-temporal regulation (reviewed in Prior et al, 2012; Stephen et al, 2014; Pylayeva-Gupta et al, 2011). Activating RAS mutations contribute to cellular proliferation, transformation and survival by activating the MAPK signaling pathway, the AKT pathway and the RAL GDS pathway, among others (reviewed in Stephen et al, 2014; Pylayeva-Gupta et al, 2011).
Although the frequency and distribution varies between RAS genes and cancer types, the vast majority of activating RAS mutations occur at one of three residues - G12, G13 and Q61. Mutations at these sites favour the RAS:GTP bound form and yield constitutively active versions of the protein (reviewed in Prior et al, 2012).