Zeaxanthin independence of photophysics in light-harvesting complex II in a membrane environment.

Green plants protect against photodamage by dissipating excess energy in a process called non-photochemical quenching (NPQ). In vivo, NPQ is activated by a drop in the luminal pH of the thylakoid membrane that triggers conformational changes of the antenna complexes, which activate quenching channels. The drop in pH also triggers de-epoxidation of violaxanthin, one of the carotenoids bound within the antenna complexes, into zeaxanthin, and so the amplitude of NPQ in vivo has been shown to increase in the presence of zeaxanthin. In vitro studies on light-harvesting complex II (LHCII), the major antenna complex in plants, compared different solubilization environments, which give rise to different levels of quenching and so partially mimic NPQ in vivo. However, in these studies both completely zeaxanthin-independent and zeaxanthin-dependent quenching have been reported, potentially due to the multiplicity of solubilization environments. Here, we characterize the zeaxanthin dependence of the photophysics in LHCII in a near-physiological membrane environment, which produces slightly enhanced quenching relative to detergent solubilization, the typical in vitro environment. The photophysical pathways of dark-adapted and in vitro de-epoxidized LHCIIs are compared, representative of the low-light and high-light conditions in vivo, respectively. The amplitude of quenching as well as the dissipative photophysics are unaffected by zeaxanthin at the level of individual LHCIIs, suggesting that zeaxanthin-dependent quenching is independent of the channels induced by the membrane. Furthermore, our results demonstrate that additional factors beyond zeaxanthin incorporation in LHCII are required for full development of NPQ.