Organization of the thylakoid membrane from the heterotrophic cyanobacterium, Aphanocapsa 6714

Abstract

The polypeptide composition of thylakoid membrane fractions from the heterotrophic cyanobacterium Aphanocapsa 6714 was examined by electrophoretic and immunoblotting procedures. We have identified thylakoid cytochromes f, b6, c-550 and c-553 by tetramethylbenzidine staining of lithium dodecyl sulfate polyacrylamide gels; we also have identified the Rieske Fe-S center protein and subunit 4 of the cytochrome b6/f complex. We have characterized phycobilisomes and active core preparations of PS I and PS II. PS I is comprised of five polypeptides (62 kDa, 14.5 kDa, 10 kDa, and two proteins of less than 10 kDa), and our PS II preparation is highly enriched for three chlorophyll-binding proteins of 48, 45 and 36 kDa. Furthermore, we have resolved the chlorophyll-binding complexes on non-denaturing gels and have determined the polypeptide composition of each chlorophyll-containing band. Three bands are associated with PS I (I, IIa and IIb) and three bands are PS II components (III', IIIa and IIIb) as judged by low-temperature fluorescence emission spectra. Band III' contains a 64 kDa antenna polypeptide, IIIa contains the 48 kDa and 45 kDa polypeptides, and IIIb is comprised solely of a 36 kDa protein. The IIIb apoprotein represents a novel PS II component; its possible role in photochemistry is discussed.