Biology subjects

Pabis, A.

Publications and source records attributed to Pabis, A..

4 recordsLinked to original sources

bioRxiv2020

Influenza hemagglutinin drives viral entry via two sequential intramembrane mechanisms

Enveloped viruses enter cells via a process of membrane fusion between the viral envelope and a cellular membrane. For influenza virus, mutational data have shown that the membrane-inserted portions of the hemagglutinin protein play a critical role in achieving fusion. In contrast to the relatively well-understood ectodomain, a predictive mechanistic understanding of the intramembrane mechanisms by which influenza hemagglutinin drives fusion has been elusive. We have used molecular dynamics simulations of fusion between a full-length hemagglutinin proteoliposome and a lipid bilayer to analyze these mechanisms. In our simulations, hemagglutinin first acts within the membrane to increase lipid tail protrusion and promote stalk formation and then acts to engage the distal leaflets of each membrane and promote stalk widening, curvature, and eventual fusion. These two sequential mechanisms, one occurring prior to stalk formation and one after, are consistent with experimental measurements we report of single-virus fusion kinetics to liposomes of different sizes. The resulting model also helps explain and integrate prior mutational and biophysical data, particularly the mutational sensitivity of the fusion peptide N-terminus and the length sensitivity of the transmembrane domain. We hypothesize that entry by other enveloped viruses may also utilize sequential processes of acyl tail exposure followed by membrane curvature and distal leaflet engagement.

biophysics↗

bioRxiv2018

Bifunctional Substrate Activation via an Arginine Residue Drives Catalysis in Chalcone Isomerases

Chalcone isomerases are plant enzymes that perform enantioselective oxa-Michael cyclizations of 2'-hydroxy-chalcones into flavanones. An X-ray crystal structure of an enzyme-product complex and molecular dynamics simulations reveal an enzyme mechanism wherein the guanidinium ion of a conserved arginine positions the nucleophilic phenoxide and activates the electrophilic enone for cyclization through Bronsted and Lewis acid interactions. The reaction terminates by asymmetric protonation of the carbanion intermediate syn to the guanidinium. Interestingly, bifunctional guanidine- and urea-based chemical reagents, increasingly used for asymmetric organocatalytic applications, are synthetic counterparts to this natural system. Comparative protein crystal structures and molecular dynamics simulations further demonstrate how two active site water molecules coordinate a hydrogen bond network that enables expanded substrate reactivity for 6'-deox-ychalcones in more recently evolved type-2 chalcone isomerases.

biochemistry↗

bioRxiv2017

Cryptic genetic variation defines the adaptive evolutionary potential of enzymes

Genetic variation among orthologous proteins can cause cryptic phenotypic properties that only manifest in changing environments. Such variation may also impact the evolutionary potential of proteins, but the molecular basis for this remains unclear. Here we perform comparative directed evolution in which four orthologous metallo-{beta}-lactamases were evolved toward a new function. We found that genetic variation between these enzymes resulted in distinct evolutionary outcomes. The ortholog with the lower initial activity reached a 20-fold higher fitness plateau exclusively via increasing catalytic activity. By contrast, the ortholog with the highest initial activity evolved to a less-optimal and phenotypically distinct outcome through changes in expression, oligomerization and activity. We show that the cryptic molecular properties and conformational variation of residues in the initial genotypes cause epistasis, thereby constraining evolutionary outcomes. Our work highlights that understanding the molecular details relating genetic variation to protein functions is essential to predicting the evolution of proteins.

evolutionary biology↗

bioRxiv2017

Evolution of chalcone isomerase from a non-catalytic ancestor

The emergence of catalysis in a non-catalytic protein scaffold is a rare, unexplored event. Chalcone isomerase (CHI), a key enzyme in plant flavonoid biosynthesis, is presumed to have evolved from a non-enzymatic ancestor related to the widely-distributed fatty-acid binding proteins (FAPs) and a plant protein family with no isomerase activity (CHILs for \"CHI-like\"). Ancestral inference confirmed that CHI evolved from a protein lacking isomerase activity. We also identified four alternative founder mutations, i.e. mutations that individually instated activity, including a mutation that is not phylogenetically traceable. Despite strong epistasis in other cases of protein evolution, CHIs laboratory reconstructed mutational trajectory shows weak epistasis. Thus, enantioselective CHI activity can readily emerge despite a catalytically inactive starting point. X-ray crystallography, NMR, and MD simulations reveal reshaping of the active site toward a productive substrate-binding mode and repositioning of the catalytic arginine that was inherited from the ancestral fatty-acid binding proteins.

biochemistry↗