Structural Characterization of Full-Length Human Dehydrodolichyl Diphosphate Synthase Using an Integrative Computational and Experimental Approach.

Dehydrodolichyl diphosphate synthase (DHDDS) is the catalytic subunit of the heteromeric human cis-prenyltransferase complex, synthesizing the glycosyl carrier precursor for N-linked protein glycosylation. Consistent with the important role of N-glycosylation in protein biogenesis, DHDDS mutations result in human diseases. Importantly, DHDDS encompasses a C-terminal region, which does not converge with any known conserved domains. Therefore, despite the clinical importance of DHDDS, our understating of its structure-function relations remains poor. Here, we provide a structural model for the full-length human DHDDS using a multidisciplinary experimental and computational approach. Size-exclusion chromatography multi-angle light scattering revealed that DHDDS forms a monodisperse homodimer in solution. Enzyme kinetics assays revealed that it exhibits catalytic activity, although reduced compared to that reported for the intact heteromeric complex. Our model suggests that the DHDDS C-terminus forms a helix-turn-helix motif, tightly packed against the core catalytic domain. This model is consistent with small-angle X-ray scattering data, indicating that the full-length DHDDS maintains a similar conformation in solution. Moreover, hydrogen-deuterium exchange mass-spectrometry experiments show time-dependent deuterium uptake in the C-terminal domain, consistent with its overall folded state. Finally, we provide a model for the DHDDS-NgBR heterodimer, offering a structural framework for future structural and functional studies of the complex.