"Disaggregation" of phytochrome in vitro-a consequence of proteolysis

The relationship between a large molecular weight (9S) and a small molecular weight (4.5S, 60,000 molecular weight) species of phytochrome was examined to determine if the larger species was an aggregate of the smaller. Alterations of pH, salt concentration, or phytochrome concentration did not cause any observable formation of the large form from the small form. However, in partially purified phytochrome extracts from Secale cereale L. and Avena sativa L., the large form was converted to the small form over time at 4 C in the dark. This breakdown was inhibitable by the protease inhibitor phenylmethanesulfonyl fluoride. When highly purified large molecular weight rye phytochrome was incubated with a neutral protease isolated from etiolated oat shoots, the large phytochrome was converted to the small form without qualitative visible absorbancy changes. The effect of the oat protease could be mimicked by a wide variety of commercial endopeptidases, including trypsin. Examination of the trypsin-induced breakdown on sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that as the size of the photoreversible unit changes from large to small, the size of its constituent polypeptide chains is reduced from 120,000 to 62,000 molecular weight. These experiments provide evidence that the endogenous breakdown observed in extracts is a result of contaminant protease and, consequently, that the small molecular weight species of phytochrome is an artifact due to proteolysis.