Characterization of the structure of the phosphoprotein of Chandipura virus, a negative stranded RNA virus probing intratryptophan energy transfer using single and double tryptophan mutants

The phosphoprotein (P protein) of Chandipura virus (CHPV), a negative stranded RNA virus, is involved in both transcription and replication phases of the viral life cycle. The two Tryptophan (Trp) residues of CHPV, located at 105 and 135 respectively and two single Trp mutants W135F and W105F and a double Trp mutant W135F/W105F have been characterized by steady state and time-resolved fluorescence and phosphorescence at 298 K and 77 K. Results indicate that Trp135 is more buried with less polar and more hydrophobic environment whereas the Trp105 is solvent exposed. Quantum yields (Ф) suggest that the singlet–singlet (S ↔ S) non-radiative energy transfer (ET) from the Trp135 to the Trp105 occurs with 66% efficiency. The simulation of the fluorescence spectra of the WT and the time resolved studies support the results. Lifetime and Ф of the single Trp mutants suggest an intrinsic static quenching of the Trp105. The results at 77 K indicate that the ET takes place from the lowest triplet state (T1) of the Trp105 to the T1 of the Trp135 apart from the backward S ↔ S ET from the Trp105 to the Trp135. The triplet–triplet (T ↔ T) ET implies a distance of <10 Å between the Trp105 and the Trp135. Using the crystal structure of Vesicular Stomatitis Virus (VSV) phosphoprotein exhibiting about 34% similarity with the CHPV P protein, a homology modelling of CHPV supports the observed distance between the Trp residues, the S ↔ S ET efficiency and the environments of the Trp residues in CHPV.