Conserved histidine residues and the control of tick-borne encephalitis virus maturation.

The maturation and infectivity of orthoflaviviruses is driven by interactions and structural changes of the two envelope proteins, prM and E, which are controlled by protonation and deprotonation events during the viral life cycle when the virus encounters acidic and neutral pH environments inside and outside the cell. Histidine residues act as sensors for these pH-induced conformational transitions, which differ between mosquito-borne and tick-borne orthoflaviviruses in some aspects. We identified six histidine residues that are conserved among the envelope proteins of mammalian tick-borne orthoflaviviruses, but not among those of mosquito-borne orthoflaviviruses. These residues might account for some of the structural differences observed between these two orthoflavivirus groups. We therefore conducted a mutational analysis by replacing each of the conserved histidine residues with alanine and determined the effect of these mutations on the maturation and infectivity of tick-borne encephalitis virus (TBEV). One of the histidine residues (H208, located in an insert unique to mammalian tick-borne orthoflaviviruses at the prM-binding site in E) was shown to be an important determinant of efficient prM cleavage. In line with recent findings regarding the infectivity of immature tick-borne orthoflaviviruses, this mutant was only slightly less infectious than the mature wild-type virus. The presence of histidine residue 208 appears to be linked to the particular requirements of the complex ecological life cycle of TBEV and likely reflects an evolutionary adaptation to the specific interactions of the virus with mammalian hosts and/or tick vectors.