High light induced accumulation of two isoforms of the CF1 α-subunit in mesophyll and bundle sheath chloroplasts of C4 plants

The effect of light irradiance on the amount of ATP synthase α-subunit in mesophyll (M) and bundle sheath (BS) chloroplasts of C4 species such as maize (Zea mays L., type NADP-ME), millet (Panicum miliaceum, type NAD-ME) and guinea grass (Panicum maximum, type PEP-CK) was investigated in plants grown under high, moderate and low light intensities equal to 800, 350 and 50 µmol photons m-2 s-1, respectively. The results demonstrate that α-subunit of ATP synthase in both M and BS chloroplasts is altered by light intensity, but differently in the investigated species. Moreover, we identified two isoforms of the CF1 α-subunit, called α and ά. The CF1 α-subunit was the major isoform and was present in all light conditions, whereas ά was the minor isoform in low light. A strong increase in the level of the ά-subunit in maize mesophyll and bundle sheath thylakoids was observed after 50 h of high light treatment. The α and ά-subunits from investigated C4 species displayed apparent molecular masses of 64 and 67 kDa, respectively, on SDS/PAGE. The presence of the ά-subunit of ATPase was confirmed in isolated CF1 complex, where it was recognized by antisera to the α-subunit. The N-terminal sequence of ά-subunit is nearly identical to that of α. Our results indicate that both isoforms coexist in M and BS chloroplasts during plant growth at all irradiances. We suggest the existence in M and BS chloroplasts of C4 plants of a mechanism(s) regulating the ATPase composition in response to light irradiance. Accumulation of the ά isoform may have a protective role under high light stress against over protonation of the thylakoid lumen and photooxidative damage of PSII.