Atomic structure of the eukaryotic intramembrane RAS methyltransferase ICMT

Abstract

The maturation of Ras GTPases, and ~200 other cellular CaaX proteins, involves three enzymatic steps: addition of a farnesyl or geranylgeranyl prenyl lipid to the cysteine (C) in the C-terminal CaaX motif, proteolytic cleavage of the aaX residues, and methylation of the exposed prenylcysteine residue at its terminal carboxylate1. This final step is catalyzed by isoprenylcysteine carboxyl methyltransferase (ICMT), a eukaryotic-specific integral membrane enzyme of the endoplasmic reticulum (ER)2. ICMT is the only cellular enzyme known to methylate prenylcysteine substrates; methylation is important for their biological functions, including the membrane localisations and subsequent activities of Ras1, prelamin A3, and Rab4. ICMT inhibition has potential for combating progeria3 and cancer5–8. Here we present an X-ray structure of ICMT, at 2.3 Å resolution, in complex with its cofactor, an ordered lipid molecule and a monobody inhibitor. The active site spans cytosolic and membrane-exposed regions, indicating distinct entry routes for its cytosolic methyl donor, S-adenosyl-L-methionine (AdoMet), and for prenylcysteine substrates, which are associated with the ER membrane. The structure suggests how ICMT overcomes the topographical challenge and unfavourable energetics of bringing two reactants that have different cellular localisations together in a membrane environment – a relatively uncharacterized, but defining feature of many integral membrane enzymes.

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“…These findings illustrate one of the mechanisms induced by pancreatitis to drive pancreatic tumorigenesis and provide an in vivo rationale to develop strategies targeting the process of KRAS prenylation. Therefore, efforts aiming to interfere with KRAS prenylation and target the cell membrane must be maintained and encouraged [ 21 , 22 , 23 , 24 ].…”

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“…It will be interesting to see whether other SBPs can inhibit nucleotide exchange in similar ways. This work on Ras/Raf has provided important insights into Ras biology, which has recently been supplemented by the development of a monobody inhibitor of isoprenylcysteine carboxyl methyltransferase, the final protein involved in the maturation of Ras [48], demonstrating that SBPs represent useful tools for targeting intracellular proteins both in vitro and in vivo, and showing that intracellular modulation of Ras is not as intransigent a problem as previously thought.…”

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