Features of Grana Organization in Pea Chloroplasts

Abstract

Two fractions of membrane fragments—the pellets precipitated at 1300 and 20000 g (fractions G1.3 and G20, respectively)—were isolated from pea (Pisum sativum L.) chloroplasts after solubilization with digitonin. These fragments assigned to grana displayed the following differences: (1) in spectra of low-temperature fluorescence, the ratio of short-wave and long-wave band intensities, as well as integrated intensity of the whole spectrum, were higher for G1.3 than for G20 fraction; (2) in excitation spectra of long-wave fluorescence, the ratio of peaks at 650 and 680 nm and integrated intensity of the spectrum were higher for G1.3 than for G20 fraction; and (3) the shapes of fluorescence excitation spectra differed for G1.3 and G20. These results indicate that the two fractions examined differed in proportion of photosystem I and photosystem II complexes, as well as in organization of these complexes. The size of light-harvesting antenna was larger in PSI complexes of G1.3 fraction, owing, in particular, to a higher content of chlorophyll a/b-protein complexes in this fraction. After repeated digitonin fragmentation of G1.3 and G20 preparations, more than 80% of G1.3 fraction was decomposed into lighter fragments, whereas G20 fraction was resistant to fragmentation (it lost about 10% of its material). Analysis of the data suggests the presence of two structurally different types of thylakoids in grana. The yield of G20 fraction (about 20%) is comparable to the ratio between the number of intergranal thylakoids, connected to granum in pea chloroplasts, and the total number of thylakoids in this granum. Based on these data, we assume that G20 fraction represent the fragments of intergranal thylakoids that extend into the granum.