Substrate-analogue complex structure of Mycobacterium tuberculosis decaprenyl diphosphate synthase

Structural-Functional Correlations between Unique N-terminal Region and C-terminal Conserved Motif in Short-chain cis-Prenyltransferase from Tomato

Neryl diphosphate (C10) synthase (NDPS1), a homodimeric soluble cis-prenyltransferase from tomato, contains four disulfide bonds, including two inter-subunit S-S bonds in the N-terminal region. Mutagenesis studies demonstrated that the S-S bond formation affects not only the stability of the dimer but also the catalytic efficiency of NDPS1. Structural polymorphs in the crystal structures of NDPS1 complexed with its substrate and substrate analog were identified by employing massive data collections and hierarchical clustering analysis. Heterogeneity of the C-terminal region, including the conserved RXG motifs, was observed in addition to the polymorphs of the binding mode of the ligands. One of the RXG motifs covers the active site with an elongated random coil when the ligands are well-ordered. Conversely, the other RXG motif was located away from the active site with a helical structure. The heterogeneous C-terminal regions suggest alternating structural transitions of the RXG motifs that result in closed and open states of the active sites. Site-directed mutagenesis studies demonstrated that the conserved glycine residue cannot be replaced. We propose that the putative structural transitions of the order/disorder of N-terminal regions and the closed/open states of C-terminal regions may cooperate and be important for the catalytic mechanism of NDPS1.

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.

Structural basis of heterotetrameric assembly and disease mutations in the human cis-prenyltransferase complex

The human cis-prenyltransferase (hcis-PT) is an enzymatic complex essential for protein N-glycosylation. Synthesizing the precursor of the glycosyl carrier dolichol-phosphate, mutations in hcis-PT cause severe human diseases. Here, we reveal that hcis-PT exhibits a heterotetrameric assembly in solution, consisting of two catalytic dehydrodolichyl diphosphate synthase (DHDDS) and inactive Nogo-B receptor (NgBR) heterodimers. Importantly, the 2.3 Å crystal structure reveals that the tetramer assembles via the DHDDS C-termini as a dimer-of-heterodimers. Moreover, the distal C-terminus of NgBR transverses across the interface with DHDDS, directly participating in active-site formation and the functional coupling between the subunits. Finally, we explored the functional consequences of disease mutations clustered around the active-site, and in combination with molecular dynamics simulations, we propose a mechanism for hcis-PT dysfunction in retinitis pigmentosa. Together, our structure of the hcis-PT complex unveils the dolichol synthesis mechanism and its perturbation in disease.

The human cis-prenyltransferase (hcis-PT) complex synthesizes the precursor of the glycosyl carrier dolichol-phosphate and as such it is essential for protein N-glycosylation. The crystal structure of the complex reveals unusual tetrameric architecture and provides insights into dolichol synthesis mechanism and functional consequences of disease-associated hcis-PT mutations.

Structural elucidation of the cis-prenyltransferase NgBR/DHDDS complex reveals insights in regulation of protein glycosylation

Cis-prenyltransferase (cis-PTase) catalyzes the rate-limiting step in the synthesis of glycosyl carrier lipids required for protein glycosylation in the lumen of endoplasmic reticulum. Here, we report the crystal structure of the human NgBR/DHDDS complex, which represents an atomic resolution structure for any heterodimeric cis-PTase. The crystal structure sheds light on how NgBR stabilizes DHDDS through dimerization, participates in the enzyme's active site through its C-terminal -RXG- motif, and how phospholipids markedly stimulate cis-PTase activity. Comparison of NgBR/DHDDS with homodimeric cis-PTase structures leads to a model where the elongating isoprene chain extends beyond the enzyme's active site tunnel, and an insert within the α3 helix helps to stabilize this energetically unfavorable state to enable long-chain synthesis to occur. These data provide unique insights into how heterodimeric cis-PTases have evolved from their ancestral, homodimeric forms to fulfill their function in long-chain polyprenol synthesis.