Sieve-tube proteins from Cucurbita maxima.

The two main proteins from the phloem exudate of Cucurbita maxima Duchesne have been isolated by ammonium-sulfate precipitation, DEAE-cellulose chromatography, and gel filtration, and have been characterized. They comprise about 40% each of the total protein. The amino-acid composition of these two proteins has been determined. Both are highly basic with an IEP above 9.5. The smaller protein has a molecular weight of ca. 30000 as determined by analytical ultracentrifugation, gel filtration, and SDS polyacrylamide gel electrophoresis. It easily dimerizes to a form which appears to be the naturally occurring structure. The larger protein has a molecular weight of 116000 (ultracentrifugation, SDS polyacrylamide gel electrophoresis). This protein forms a gel in the absence of SH-protecting agents. Several minor proteins have been detected by polyacrylamide gel electrophoresis.