Abstract
Chloroplast interior contains thylakoid membranes, the stacked and unstacked regions of which are called the grana and stroma lamella, respectively. Photosystem I (PSI) predominantly exists in the stroma lamella, and photosystem II and its light-harvesting antenna proteins (LHCII) usually exist in the grana. Recent biochemical studies show that LHCII optimizes the rate of photochemistry by physically shuttling between the two photosystems. However, it is still unknown how LHCII migrates along thylakoid membranes due to the lack of direct observation. In this study, we directly observed the photosynthetic membrane proteins in the isolated thylakoid membranes by using atomic force microscopy. The results indicated that most part of the thylakoid membranes were crowded by membrane proteins. This will draw attention to reconsider how LHCII migrates in the crowded membranes to counterbalance the energy level of each photosystem during photoacclimation in a short period of time.
