Differential functions of the KRAS splice variants

Abstract

RAS proteins are small GTPases that transduce signals from membrane receptors to signaling pathways that regulate growth and differentiation. Four RAS proteins are encoded by three genes - HRAS, KRAS, NRAS. Among them, KRAS is mutated in human cancer more frequently than any other oncogene. The KRAS pre-mRNA is alternatively spliced to generate two transcripts, KRAS4A and KRAS4B, that encode distinct proto-oncoproteins that differ almost exclusively in their C-terminal hypervariable regions (HVRs) that controls subcellular trafficking and membrane association. The KRAS4A isoform arose 475 million years ago in jawed vertebrates and has persisted in all vertebrates ever since, strongly suggesting non-overlapping functions of the splice variants. Because KRAS4B is expressed at higher levels in most tissues, it has been considered the principal KRAS isoform. However, emerging evidence for KRAS4A expression in tumors and splice variant-specific interactions and functions have sparked interest in this gene product. Among these findings, the KRAS4A-specific regulation of hexokinase I is a stark example. The aim of this mini-review is to provide an overview of the origin and differential functions of the two splice variants of KRAS.

Keywords: KRAS; KRAS4A; KRAS4B; alternative splicing; evolution; oncogene.

Figure 1.. Differential posttranslational processing regulates KRAS isoform subcellular trafficking and interactome.

The trafficking and interactome of KRAS isoforms KRAS4A (blue) and KRAS4B (green) are schematized. For both KRAS4A and KRAS4B, (+) signs indicate polybasic residues that confer electrostatic affinity for negatively charged phospholipid headgroups. KRAS4B is phosphorylated by PKC (dark green) leading to a significant redistribution to endomembrane compartments, including the ER [27]. KRAS4B interacts with v-ATPase 2 on the cytosolic face of lysosomal membranes [35]. Additionally, KRAS4B is a client of the prenyl-binding protein PDE6δ (pink) that shuttles prenylated proteins between membrane compartments [32]. On the other hand, KRAS4A undergoes a cycle of palmitoylation/depalmitoylation on cysteine 180 that drives association with the plasma membrane or outer mitochondrial membrane (OMM), respectively. KRAS4A interacts with HK1 on the OMM in a GTP-dependent fashion and stimulates HK1 activity [21]. The subcellular distribution of KRAS4A also depends on SIRT2 deacylase activity leading to redistribution of KRAS4A onto endomembranes increasing the affinity to ARAF (green) [20]. Farnesylation, palmitoylation and transport of the proteins are depicted in the figure legend. Created with BioRender.com.

Figure 2.. Scheme of oncoprotein expansion from cephalochordates to mammals.

Oncogene exon–intron structures are depicted with exons as boxes (1–4 coding, 0 and 0’ non-coding) and introns as lines connecting the exons (a–d). Modified from [1]. Created with BioRender.com.