Role of Cysteines in Stabilizing the Randomized Receptor Binding Domains within Feline Leukemia Virus Envelope Proteins.

Retargeting of gammaretroviral envelope proteins has shown promising results in the isolation of novel isolates with therapeutic potential. However, the optimal conditions required to obtain high-affinity retargeted envelope proteins with narrow tropism are not understood. This study highlights the advantage of constrained peptides within receptor binding domains and validates the random library screening technique of obtaining novel retargeted Env proteins. Using a modified vector backbone to screen the envelope libraries on 143B osteosarcoma cells, three novel and unique retargeted envelopes were isolated. The use of complex disulfide bonds within variable regions required for receptor binding is found within natural gammaretroviral envelope isolates. Interestingly, two of the isolates, named AII and BV2, have a pair of cysteines located within the randomized region of 11 amino acids similar to that identified within the CP Env, an isolate identified in a previous Env library screen on the human renal carcinoma Caki-1 cell line. The amino acids within the randomized region of AII and BV2 envelopes that are essential for viral infection have been identified in this study and include these cysteine residues. Through mutagenesis studies, the putative disulfide bond pairs including and beyond the randomized region were examined. In parallel, the disulfide bonds of CP Env were identified using mass spectrometry. The results indicate that this pair of cysteines creates the structural context to position key hydrophobic (F and W) and basic (K and H) residues critical for viral titer and suggest that AII, BV2, and CP internal cysteines bond together in distinct ways. Retargeted gammaretroviral particles have broad applications for therapeutic use. Although great advances have been achieved in identifying new Env-host cell receptor pairs, the rules for designing optimal Env libraries are still unclear. We have found that isolates with an additional pair of cysteines within the randomized region have the highest transduction efficiencies. This emphasizes the importance of considering cysteine pairs in the design of new libraries. Furthermore, our data clearly indicate that these cysteines are essential for viral infectivity by presenting essential residues to the host cell receptor. These studies facilitate the screening of Env libraries for functional entry into target cells, allowing the identification of novel gammaretroviral Envs targeting alternative host cell receptors for gene and protein delivery.