The light-harvesting chlorpohyll-protein complex of photosystem II. Its location in the photosynthetic membrane.

Abstract

The structure of the photosynthetic membrane in a mutant of barley known to lack a chlorophyll-binding protein was investigated. This protein probably channels excitation energy to photosystem II, and is known as the light-harvesting chlorophyll-protein complex. Extensive stacking of thylakoids into grana occurs in mutant and wild-type chloroplasts. Examination of membrane internal structure by freeze-fracturing indicates that only slight differences exist between the fracture faces of mutant and wild-type membranes. These differences are slight reductions in the size of particles visible on the EFs [thylakoid fracture face according to the system of Staehelin] fracture face, and in the number of particles seen on the PFs [thylakoid fracture face according to the system of Staehelin] fracture face. No differences can be detected between mutant and wild-type on the etched outer surface of the membrane. In contrast, tetrameric particles visible on the etched inner surface of wild-type thylakoids are extremely difficult to recognize on similar surfaces of the mutant. These particles can be recognized on inner surfaces of the mutant membranes when they are organized into regular lattices, but these lattices show a much closer particle-to-particle spacing than similar lattices in wild-type membranes. Although several interpretations of these data are possible, these observations are consistent with the proposal that the light-harvesting chlorophyll-protein complex of photosystem II is bound to the tetramer (which is visible on the EFs face as a single particle) near the inner surface of the membrane. The large tetramer, which other studies have shown to span the thylakoid membrane, may represent an assembly of protein, lipid, and pigment comprising all the elements of the photosystem II reaction. A scheme is presented which illustrates 1 possibility for the integration of the light reaction across the photosynthetic membrane.