Protein-Based Model for Energy Transfer between Photosynthetic Light-Harvesting Complexes Is Constructed Using a Direct Protein-Protein Conjugation Strategy

Photosynthetic organisms utilize dynamic and complexnetworks ofpigments bound within light-harvesting complexes to transfer solarenergy from antenna complexes to reaction centers. Understanding theprinciples underlying the efficiency of these energy transfer processes,and how they may be incorporated into artificial light-harvestingsystems, is facilitated by the construction of easily tunable modelsystems. We describe a protein-based model to mimic directional energytransfer between light-harvesting complexes using a circular permutantof the tobacco mosaic virus coat protein (cpTMV), which self-assemblesinto a 34-monomer hollow disk. Two populations of cpTMV assemblies,one labeled with donor chromophores and another labeled with acceptorchromophores, were coupled using a direct protein-protein bioconjugationmethod. Using potassium ferricyanide as an oxidant, assemblies containing o-aminotyrosine were activated toward the addition of assembliescontaining p-aminophenylalanine. Both of these noncanonicalamino acids were introduced into the cpTMV monomers through ambercodon suppression. This coupling strategy has the advantages of directly,irreversibly, and site-selectively coupling donor with acceptor proteinassemblies and avoids cross-reactivity with native amino acids andundesired donor-donor or acceptor-acceptor combinations.The coupled donor-acceptor model was shown to transfer energyfrom an antenna disk containing donor chromophores to a downstreamdisk containing acceptor chromophores. This model ultimately providesa controllable and modifiable platform for understanding photosyntheticinterassembly energy transfer and may lead to the design of more efficientfunctional light-harvesting materials.