Molecular weight, size and shape of thylakoid-membrane proteins

Abstract

Chloroplasten, Membranproteine, Molekulargewicht, Konformation The lamellar system of chloroplasts, dissolved in concentrated formic acid, may be freed from lipids by dialysis or gel filtration. The structural protein on both sides of the isoelectric region is water-soluble. The molecular weight was determined to be 470 000 at pH 3.0 - 3.4. Electron micrographs reveal disc-like particles of variable diameter but constant thickness. Particles of 100 Å diameter and 50 Å thickness are mainly observed. The limiting viscosity number [η] = 12 (ml/g) demonstrated that the particles are also anisodiametric in solution. It follows from changes in circular dichroism and infrared absorption that the conformation of the polypeptides is altered by the formic acid treatment. The structural protein can be separated into polypeptides in buffers containing dodecylsulphate, yielding a major component of molecular weight 25 000. From these experiments it may be concluded that one protein layer is contained in the thylakoid membrane which is composed of supra-molecular structural elements.