The proteins of green leaves. IV. A high molecular weight protein comprising a large part of the cytoplasmic proteins.

Previous work has shown that the soluble cytoplasmic proteins comprise about 30 per cent of the total protein contained in spinach and tobacco leaves (1, 2). Thus, on a dry weight basis, the cytoplasmic proteins constitute in the neighborhood of 10 per cent of the total dry matter of the leaf, including cellulose. In an attempt to resolve the soluble proteins present in leaf cytoplasm, resort was made to electrophoretic analysis. The results indicated that most of the cytoplasmic proteins migrated as a single boundary under the conditions studied. Although the shapes of the scanning patterns were sufficiently asymmetrical to suggest some heterogeneity among the cytoplasmic proteins, nevertheless the impression was gained from electrophoresis and other experiments that a substantial portion of the soluble cytoplasmic proteins consisted of a single protein component with respect to mobility. Ultracentrifugal analysis, however, achieved better resolution of this mixture, and examination by this method of the cytoplasmic proteins extracted from several species of plant leaves reveals clearly the presence of a high molecular weight component comprising from 23 to 50 per cent of the total cytoplasmic proteins in each of the cytoplasms investigated. Moreover, the behavior of this component during ultracentrifugation suggests a high degree of homogeneity, with respect to molecular weight. It therefore appears that an appreciable part of the total dry weight of the leaf can be considered as consisting of a single kind of protein, which, in accord with our previous usage, we shall call Fraction I protein. A preliminary report of this work has been presented (3).