Topology Dictates Evolution of Regulatory Cysteines in a Family of Viral Oncoproteins

ALVAREZ-PAGGI, DAMIÁN; LORENZO, JUAN RAMIRO; CAMPOREALE, GABRIELA; MONTERO, LUCIANO; SÁNCHEZ, IGNACIO E; DE PRAT GAY, GONZALO; ALONSO, LEONARDO G

MOLECULAR BIOLOGY AND EVOLUTION

OXFORD UNIV PRESS

Año: 2019 vol. 36 p. 1521 - 1532

0737-4038

Redox regulation in biology is largely operated by cysteine chemistry in response to a variety of cell environmental andintracellular stimuli. The high chemical reactivity of cysteines determines their conservation in functional roles, but theirpresence can also result in harmful oxidation limiting their general use by proteins. Papillomaviruses constitute a uniquesystem for studying protein sequence evolution since there are hundreds of anciently evolved stable genomes. E7, theviral transforming factor, is a dimeric, cysteine-rich oncoprotein that shows both conserved structural and variableregulatory cysteines constituting an excellent model for uncovering the mechanism that drives the acquisition of redoxsensitive groups. By analyzing over 300 E7 sequences, we found that although noncanonical cysteines show no obvioussequence conservation pattern, they are nonrandomly distributed based on topological constrains. Regulatory residuesare strictly excluded from six positions stabilizing the hydrophobic core while they are enriched in key positions locatedat the dimerization interface or around the Znþ2 ion. Oxidation of regulatory cysteines is linked to dimer dissociation,acting as a reversible redox-sensing mechanism that triggers a conformational switch. Based on comparative sequenceanalysis, molecular dynamics simulations and biophysical analysis, we propose a model in which the occurrence ofcysteine-rich positions is dictated by topological constrains, providing an explanation to why a degenerate pattern ofcysteines can be achieved in a family of homologs. Thus, topological principles should enable the possibility to identifyhidden regulatory cysteines that are not accurately detected using sequence based methodology