Functional consequence of mutating conserved residues of the yeast farnesyl-protein transferase beta-subunit Ram1(Dpr1)

Hydrophobic handoff for direct delivery of peroxisome tail-anchored proteins

Tail-anchored (TA) proteins are inserted into membranes post-translationally through a C-terminal transmembrane domain (TMD). The PEX19 protein binds peroxisome TA proteins in the cytoplasm and delivers them to the membrane through the PEX3 receptor protein. An amphipathic segment in PEX19 promotes docking on PEX3. However, how this leads to substrate insertion is unknown. Here we reconstitute peroxisome TA protein biogenesis into two sequential steps of substrate TMD engagement and membrane insertion. We identify a series of previously uncharacterized amphipathic segments in PEX19 and identify one whose hydrophobicity is required for membrane insertion, but not TMD chaperone activity or PEX3 binding. A membrane-proximal hydrophobic surface of PEX3 promotes an unconventional form of membrane intercalation, and is also required for TMD insertion. Together, these data support a mechanism in which hydrophobic moieties in the TMD chaperone and its membrane-associated receptor act in a concerted manner to prompt TMD release and membrane insertion.

A mutant form of human protein farnesyltransferase exhibits increased resistance to farnesyltransferase inhibitors

Protein farnesyltransferase (FTase) is a key enzyme responsible for the lipid modification of a large and important number of proteins including Ras. Recent demonstrations that inhibitors of this enzyme block the growth of a variety of human tumors point to the importance of this enzyme in human tumor formation. In this paper, we report that a mutant form of human FTase, Y361L, exhibits increased resistance to farnesyltransferase inhibitors, particularly a tricyclic compound, SCH56582, which is a competitive inhibitor of FTase with respect to the CAAX (where C is cysteine, A is an aliphatic amino acid, and X is the C-terminal residue that is preferentially serine, cysteine, methionine, glutamine or alanine) substrates. The Y361L mutant maintains FTase activity toward substrates ending with CIIS. However, the mutant also exhibits an increased affinity for peptides terminating with CIIL, a motif that is recognized by geranylgeranyltransferase I (GGTase I). The Y361L mutant also demonstrates activity with Ha-Ras and Cdc42Hs proteins, substrates of FTase and GGTase I, respectively. In addition, the Y361L mutant shows a marked sensitivity to a zinc chelator HPH-5 suggesting that the mutant has altered zinc coordination. These results demonstrate that a single amino acid change at a residue at the active site can lead to the generation of a mutant resistant to FTase inhibitors. Such a mutant may be valuable for the study of the effects of FTase inhibitors on tumor cells.