Structural homology between semidwarfism-related proteins and glutelin seed protein in rice (Oryza sativa L.)

Two semidwarfism-related proteins, SRP-1 and SRP-2, were detected as major spots in a long-culm rice cultivar, Norin 29 and its semidwarf near-isogenic line, SC-TN1, respectively, by two-dimensional gel electrophoresis (2D-PAGE). The testcross showed that SRP-1 and SRP-2 are controlled by codominant alleles, Srp-1 and Srp-2, respectively, at a single locus Srp. This locus was considered to be closely linked with the semidwarfing gene locus sd-1. SRP-1 and SRP-2 were separated by 2D-PAGE, electroblotted onto a polyvinylidene difluoride membrane, and sequenced by a gas-phase protein sequencer. The N-terminal amino acid sequences, however, could not be determined due to the blockage of the N-terminals of these proteins. After removal of the N-terminal residue with pyroglutamyl peptidase given to the membrane, the amino acid sequence in the N-terminal region was determined. The N-terminal and internal amino acid sequences of SRP-1 and SRP-2 were highly homologous with those of the glutelin α-subunits of seed endosperm storage protein, which were deduced by the cDNA sequences. In the seed endosperms of Norin 29 and SC-TN1, a total of eight glutelin α-subunits was identified by 2D-PAGE. The amino acid sequences in the N-terminal and internal regions of these proteins were determined. This experiment confirmed that SRP-1 and SRP-2 are almost identical in structure with the glutelin α5a- and α5b-subunits, respectively, which were identified in several organs such as endosperms, embryos, and leaves, unlike the other glutelin α-subunits.