Sep2, a light-harvesting complex-like protein, is involved in light stress response by binding to free chlorophylls

The light-harvesting complex (LHC) protein superfamily includes the LHC family and LHC-like family, each of which contains one or two conserved chlorophyll (Chl)-binding motifs. The LHC-like family includes early light-inducible proteins (Elips), light harvesting-like3 (Lil3s), one-helix proteins (Ohps), and stress-enhanced proteins (Seps). Currently, only the Chl-binding ability and biological function of Elips, Lil3s, and Ohps have been reported. However, the role of Seps remains unknown. In the present study, we investigated and compared the functions of two Arabidopsis (Arabidopsis thaliana) Sep proteins, Sep1 and Sep2, in Chl breakdown and in response to light stress. Our results showed that Sep1 and Sep2 contained a conserved Chl-binding motif and were localized within chloroplasts. The expression of Sep1 and Sep2 in leaves was mainly induced by light stress. Overexpression of Sep2 caused a decrease in the levels of Chl by promoting Chl breakdown, whereas Sep1 overexpression did not. Moreover, reduced accumulation of reactive oxygen species was observed in the Sep2-overexpressing Arabidopsis leaves compared with that in wild-type leaves under long-term light stress. The Sep2-induced Chl breakdown was compromised when the conserved residue (glutamic acid at position 105) of the Chl-binding motif was mutated. However, neither wild-type Sep1 nor mutant Sep1 caused Chl breakdown in Arabidopsis leaves. These findings suggested that Sep2, as a member of the LHC-like family, might regulate the levels of free Chl by binding to Chl, thereby participating in light-stress response.