Mutants for rice storage proteins : 2. Isolation and characterization of protein bodies from rice mutants

Rice storage proteins of the endosperm are localized in two types of protein bodies, PB-I and PB-II. Protein bodies were isolated by sucrose density gradient centrifugation from developing endosperm of three rice mutants, CM 21, CM 1675 and CM 1834, and characterized after pepsin-digestion treatment by protein contents determination. Mutant protein bodies (PBs) except for their internal structure, were similar in shape and density to PB-I of the variety Kinmaze. Electrophoretic analysis of PB-I polypeptides revealed that SDS (Sodium dodecylsulfate) bands of 13 and 16 kilodaltons consisted, respectively, of four and two individual polypeptides with different pI values, while the 10-kilodalton band behaved as a single polypeptide after isoelectric focussing (IEF) electrophoresis. The differences in the polypeptide composition induced by mutants were due to the decrease and/or increase in the content of specific PB-I polypeptides. Electron microscopic observations revealed that the typical lamellar structure of the PB-I is not visible in CM 1675. On the contrary, the inner portion of PB-I in CM 1834 and CM 21 showed higher electron density than that of the variety Kinmaze. On these two mutants, the content of pepsin-indigestible and -digestible proteins were similar to those of Kinmaze, although the values of the PB-II/PB-I ratio were greater than those for Kinmaze, suggesting that these two mutants are high-glutelin rice mutants.