Cloning and functional analysis of cis-prenyltransferase from Thermobifida fusca

Crystal structure of Thermobifida fusca cis-prenyltransferase reveals the dynamic nature of its RXG motif-mediated inter-subunit interactions critical for its catalytic activity

cis-Prenyltransferases (cis-PTs) catalyze consecutive condensations of isopentenyl diphosphate to an allylic diphosphate acceptor to produce a linear polyprenyl diphosphate of designated length. Dimer formation is a prerequisite for cis-PTs to catalyze all cis-prenyl condensation reactions. The structure-function relationship of a conserved C-terminal RXG motif in cis-PTs that forms inter-subunit interactions and has a role in catalytic activity has attracted much attention. Here, we solved the crystal structure of a medium-chain cis-PT from Thermobifida fusca that produces dodecaprenyl diphosphate as a polyprenoid glycan carrier for cell wall synthesis. The structure revealed a characteristic dimeric architecture of cis-PTs in which a rigidified RXG motif of one monomer formed inter-subunit hydrogen bonds with the catalytic site of the other monomer, while the RXG motif of the latter remained flexible. Careful analyses suggested the existence of a possible long-range negative cooperativity between the two catalytic sites on the two monomeric subunits that allowed the binding of one subunit to stabilize the formation of the enzyme-substrate ternary complex and facilitated the release of Mg-PPi and subsequent intra-molecular translocation at the counter subunit so that the condensation reaction could occur in consecutive cycles. The current structure reveals the dynamic nature of the RXG motif and provides a rationale for pursuing further investigations to elucidate the inter-subunit cooperativity of cis-PTs.

Substrate specificity of undecaprenyl diphosphate synthase from the hyperthermophilic archaeon Aeropyrum pernix

Cis-prenyltransferase from a hyperthermophilic archaeon Aeropyrum pernix was expressed in Escherichia coli and purified for characterization. Properties such as substrate specificity, product chain-length, thermal stability and cofactor requirement were investigated using the recombinant enzyme. In particular, the substrate specificity of the enzyme attracts interest because only dimethylallyl diphosphate and geranylfarnesyl diphosphate, both of which are unusual substrates for known cis-prenyltransferases, are likely available as an allylic primer substrate in A. pernix. From the enzymatic study, the archaeal enzyme was shown to be undecaprenyl diphosphate synthase that has anomalous substrate specificity, which results in a preference for geranylfarnesyl diphosphate. This means that the product of the enzyme, which is probably used as the precursor of the glycosyl carrier lipid, would have an undiscovered structure.

Identification and characterization of a cis,trans-mixed heptaprenyl diphosphate synthase from Arabidopsis thaliana

In eukaryotes, dolichols (C(70-120)) play indispensable roles as glycosyl carrier lipids in the biosynthesis of glycoproteins on endoplasmic reticulum. In addition to dolichols, seed plants have other types of Z,E-mixed polyisoprenoids termed ficaprenol (tri-trans,poly-cis-polyprenol, C(45-75)) and betulaprenol (di-trans,poly-cis-polyprenol, C(30-45) and C(≥70)) in abundance. However, the physiological significance of these polyprenols has not been elucidated because of limited information regarding cis-prenyltransferases (cPTs) which catalyze the formation of the structural backbone of Z,E-mixed polyisoprenoids. In the comprehensive identification and characterization of cPT homologues from Arabidopsis thaliana, AtHEPS was identified as a novel cis,trans-mixed heptaprenyl diphosphate synthase. AtHEPS heterologously expressed in Escherichia coli catalyzed the formation of C(35) polyisoprenoid as a major product, independent of the chain lengths of all-trans allylic primer substrates. Kinetic analyses revealed that farnesyl diphosphate was the most favorable for AtHEPS among the allylic substrates tested suggesting that AtHEPS was responsible for the formation of C(35) betulaprenol. AtHEPS partially suppressed the phenotypes of a yeast cPT mutant deficient in the biosynthesis of dolichols. Moreover, in A. thaliana cells, subcellular localization of AtHEPS on the endoplasmic reticulum was shown by using green fluorescent protein fused proteins. However, a cold-stress-inducible expression of AtHEPS suggested that AtHEPS and its product might function in response to abiotic stresses rather than in cell maintenance as a glycosyl carrier lipid on the endoplasmic reticulum.