Structural insights into the allosteric site of Arabidopsis NADP-malic enzyme 2: role of the second sphere residues in the regulatory signal transmission.

KEY MESSAGE: NADP-ME2 from Arabidopsis thaliana exhibits a distinctive and complex regulation by fumarate, acting as an activator or an inhibitor according to substrate and effector concentrations. In this work, we used molecular modeling approach and site-directed mutagenesis to characterized the NADP-ME2 structural determinants necessary for allosteric regulation providing new insights for enzyme optimization. Structure-function studies contribute to deciphering how small modifications in the primary structure could introduce desirable characteristics into enzymes without affecting its overall functioning. Malic enzymes (ME) are ubiquitous and responsible for a wide variety of functions. The availability of a high number of ME crystal structures from different species facilitates comparisons between sequence and structure. Specifically, the structural determinants necessary for fumarate allosteric regulation of ME has been of particular interest. NADP-ME2 from Arabidopsis thaliana exhibits a distinctive and complex regulation by fumarate, acting as an activator or an inhibitor according to substrate and effector concentrations. However, the 3D structure for this enzyme is not yet reported. In this work, we characterized the NADP-ME2 allosteric site by structural modeling, molecular docking, normal mode analysis and mutagenesis. The regulatory site model and its docking analysis suggested that other C4 acids including malate, NADP-ME2 substrate, could also fit into fumarate's pocket. Besides, a non-conserved cluster of hydrophobic residues in the second sphere of the allosteric site was identified. The substitution of one of those residues, L62, by a less flexible residue as tryptophan, resulted in a complete loss of fumarate activation and a reduction of substrate affinities for the active site. In addition, normal mode analysis indicated that conformational changes leading to the activation could originate in the region surrounding L62, extending through the allosteric site till the active site. Finally, the results in this work contribute to the understanding of structural determinants necessary for allosteric regulation providing new insights for enzyme optimization.

Using normal mode analysis on protein structural models. How far can we go on our predictions?

Normal mode analysis (NMA) is a fast and inexpensive approach that is largely used to gain insight into functional protein motions, and more recently to create conformations for further computational studies. However, when the protein structure is unknown, the use of computational models is necessary. Here, we analyze the capacity of NMA in internal coordinate space to predict protein motion, its intrinsic flexibility, and atomic displacements, using protein models instead of native structures, and the possibility to use it for model refinement. Our results show that NMA is quite insensitive to modeling errors, but that calculations are strictly reliable only for very accurate models. Our study also suggests that internal NMA is a more suitable tool for the improvement of structural models, and for integrating them with experimental data or in other computational techniques, such as protein docking or more refined molecular dynamics simulations.

Genetic and structural analysis of Merkel cell polyomavirus large T antigen from diverse biological samples.

Merkel cell polyomavirus (MCPyV) large T antigen (LT-ag) is frequently found truncated in Merkel cell carcinomas (MCC) and it is considered a major tumor-specific signature. Nonetheless, the biological role of LT-ag nontruncated mutations is largely unknown. In this study, MCPyV LT-ag second exon from 11 non-MCC oral samples and NCBI sequences derived from different anatomical sites were studied from the genetic and structural standpoint. As expected, the LT-ag mutation profile was influenced by the geographical origin of the sample, although nonsynonymous mutations were more frequent in lesional tissues. Our in silico study suggests that the mutations found would not significantly affect protein functions, regardless of sample category. This work presents a thorough investigation of the structural and functional properties of LT-ag nontruncated mutations in MCPyV. Our results sustain the geographical influence of the MCPyV genetic profile, but do not discard genetic tissue specificities. Further investigation involving other genetic segments in healthy and lesional tissues are necessary to improve our knowledge on MCPyV pathogenesis.