Sb. Long et al., The basis for K-Ras4B binding specificity to protein farnesyl-transferase revealed by 2 angstrom resolution ternary complex structures, STRUCT F D, 8(2), 2000, pp. 209-222
Background: The protein farnesyltransferase (FTase) catalyzes addition of t
he hydrophobic farnesyl isoprenoid to a cysteine residue fourth from the: C
terminus of several protein accepters that are essential for cellular sign
al transduction such as Ras and Rho, This addition is necessary for the bio
logical function of the modified proteins. The majority of Ras-related huma
n cancers are associated with oncogenic variants of K-RasB, which is the hi
ghest affinity natural,substrate of FTase, Inhibition of FTase causes regre
ssion of Ras-mediated tumors in animal models.
Results: We present four ternary complexes of rat FTase cc-crystallized wit
h farnesyl diphosphate analogs and K-Ras4B peptide substrates, The Ca(1)a(2
)X portion of the peptide substrate binds in an extended conformation:in th
e hydrophobic cavity of FTase and coordinates the active site zinc ion, The
se complexes offer the first view of the polybasic region of the K-Ras4B pe
ptide substrate, which confers the major enhancement of affinity of this su
bstrate. The polybasic region forms a type I beta turn and binds along the
rim of the hydrophobic cavity, Removal of the catalytically essential zinc
ion results ina dramatically different peptide conformation in which the Ca
(1)a(2)X motif adopts a beta turn. A-manganese ion binds to the diphosphate
mimic of the farnesyl diphosphate analog.
Conclusions: These ternary complexes provide new insight into the molecular
basis of peptide substrate specificity, and further define the roles of zi
nc and magnesium in the prenyltransferase reaction. Zinc is essential for p
roductive Ca(1)a(2)X peptide binding, suggesting that the p-turn conformati
on identified in previous nuclear magnetic resonance (NMR) studies reflects
a state in which the cysteine is not coordinated to the zinc ion. The stru
ctural information presented here should facilitate structure-based design
and optimization of inhibitors of Ca(1)a(2)X protein prenyltransferases.