Fig. 4: The organization of the soluble cryptophyte light harvesting antenna in the thylakoid lumen.

a Electron micrograph of a thin section from a whole freeze-substituted H. andersenii cell (overview). b Close up of the chloroplast showing clear striations of dense material in the thylakoid lumen. c A model of the soluble light harvesting antenna residing in the lumenal space of the thylakoid. HaPE645 (purple) resides near the membrane while the lumenal space is filled with a dense phase of HaPE555 and HaPE560 (red and pink, respectively). A typical energy transfer path is shown as a series of black lines connecting PBPs to the integral membrane photosystems (green). The capital letters and numbers along this path refer to energy transfer processes as described in f. d The absorption spectrum of the major soluble cryptophyte antenna protein HaPE555 (red) plugs a spectra hole in the absorption spectrum of the photosystems and antennas in the thylakoid membrane (green). e The fluorescence emission of HaPE555 (red) shows minimal spectral overlap with the absorption spectrum of the thylakoid membrane system (green) where chlorophyll is the main chromophore in the integral membrane antennas and photosystems. f Proposed light harvesting pathway shown as a series of excitation-emission maps (A-C) demonstrating internal excitation transfer within PBPs with associated 1D spectra showing overlap between emission and excitation (2–4) for inter molecular energy transfer between proteins. This pathway corresponds to that shown in c. Step 1: sunlight is incident on HaPE555 which absorbs around 551 nm. Energy transfer moves the excitation within the protein (excitation-emission map A). Step 2: energy is transferred to either HaPE555 or HaPE560 as the emission spectra of HaPE555/HaPE560 have substantial spectral overlap with the absorption spectra of HaPE555/HaPE560 (spectrum 2 shows overlap of emission of HaPE555 (red) and absorption of HaPE560 (pink)). The excitation moves within HaPE560 (excitation-emission map B). Step 3: energy is transferred from either HaPE555 or HaPE560 to HaPE645 as their emission spectra 3 (red, only HaPE555 shown) have substantial spectral overlap with the absorption spectrum of HaPE645 (purple). Within HaPE645, the excitation migrates to the final acceptor PCB β82 chromophore (excitation-emission map C). Step 4: spectral overlap between HaPE645 emission (purple) and photosystem absorption (green) facilitates energy transfer to the integral membrane system. The location of the HaPE645 adaptor that couples the soluble antenna to the integral membrane systems must be close to the membrane for efficient transport (c).