The origin of the long-wavelength fluorescence emission band (77°K) from photosystem I

Abstract

Isolated photosystem I (PSI)-110 particles, prepared using a minimal concentration of Triton X-100 [ J. E. Mullet, J. J. Burke, and C. J. Arntzen (1980) Plant Physiol. 65, 814–822 ] and further subjected to short-term solubilization with sodium dodecyl sulfate (SDS), were resolved into four pigment-containing bands on polyacrylamide gel electrophoresis (PAGE). We have identified these in order of increasing electrophoretic mobility as being (a) CPIa, (b) CPI, (c) the light-harvesting complex of photosystem I (LHC-I), and (d) a free pigment-zone. LHC-I had an absorption maximum in the red at 668–669 nm and a shoulder at 650 nm, which was resolved by its first-derivative spectrum to indicate the presence of chlorophyll b. LHC-I exhibited a 77 °K fluorescence emission maximum at 729–730 nm. The 77°K fluorescence emission maxima of CPIa and CPI, excised from the gel, were at 729 and 722 nm, respectively. The LHC-I band, excised from the gel and rerun on dissociating SDS-PAGE, was resolved into two polypeptide doublets of 24-22.5 and 21-20.5 kDa. The CPIa band under similar conditions was resolved into polypeptides of 68, 24, 22.5, 21, 20.5, 19, 15, and 14 kDa; on the contrary, CPI contained only the 68-kDa polypeptide. When intact thylakoids were subjected to “nondenaturing” SDS-PAGE, LHC-I comigrated with an oligomeric form (dimer) of the light-harvesting chlorophyll a/b pigment-protein that preferentially serves photosystem II (LHCP-II). When this combined LHC-I/ LHCP-II pigment-protein band was prepared by SDS-PAGE from isolated stroma lamellae, it exhibited a long-wavelength fluorescence band near 730 nm at 77 °K. When a similar preparation was obtained from sucrose density gradients containing SDS [ J. Argyroudi-Akoyunoglou and H. Thomou (1981) FEBS Lett. 135, 171–181 ], it was found to be enriched in a 21-kDa polypeptide. The data suggest that the 21-kDa polypeptide of LHC-I is the chlorophyll-containing polypeptide responsible for the long-wavelength fluorescence of LHC-I; other polypeptides in the complex (20.5, 22.5, and 24 kDa) presumably bind chlorophyll and also serve an antennae function.