Research

The Johnson Lab is focused on photosynthesis the process that uses solar energy to transform water and carbon dioxide into the energy we consume and the oxygen we breathe. Research within my group falls into three overlapping areas:

Thylakoid membrane structure and dynamics


The chloroplast thylakoid membrane is the site for the initial steps of photosynthesis that convert solar energy into chemical energy, ultimately powering almost all life on earth. The heterogeneous distribution of protein complexes within the membrane gives rise to an intricate three-dimensional structure that is nonetheless extremely dynamic on a timescale of seconds to minutes. These dynamics form the basis for the regulation of photosynthesis, and therefore the adaptability of plants to different environments. We use a multi-faceted approach that includes atomic force, electron and fluorescencemicroscopies in combination with biochemistry and spectroscopy to probe these organizational details and understand their functional relevance.

Electron transfer dynamics


Small diffusible redox proteins play a ubiquitous role in facilitating electron transfer (ET) in respiration and photosynthesis by shuttling electrons between membrane bound complexes in a redox-dependent manner. The association of such small redox carrier proteins with their larger membrane-bound partners must be highly specific, yet also readily reversible in order to sustain rapid ET and turnover on microsecond/millisecond timescales. New developments in force spectroscopy provide the first opportunity to quantify the dynamic forces that sustain these transient interactions and to understand their temporal evolution leading to dissociation. We haveadapted a new atomic force microscopy (AFM) technique PeakForce-QNM (PF-QNM), and have used it to directly monitor these intermolecular interactions at the single molecule level, with sub-millisecond time resolution, and with picoNewton force resolution.

Non-photochemical quenching


In photosynthesis, light-harvesting complexes (LHCs) capture solar energy and feed it to the downstream molecular machinery. However, when light absorption exceeds the capacity for utilization, the excess energy can cause damage. Thus, LHCs have evolved a feedback loop that triggers photoprotective energy dissipation. The critical importance of photoprotection for plant fitness has been demonstrated, as well as its impact on crop yields. However, the mechanisms of photoprotection ― from fast chemical reactions of molecules to slow conformational changes of proteins ― have not yet been resolved. We are working with partners at MIT and at Okazaki in Japan to study the protein and pigment dynamics that bring about the switch between the photoprotective and light harvesting states.