| Abstract: | Major intrinsic proteins (MIPs) are believed to contribute towards the maintenance of structural integrity, osmoregulation, and responses to water and salt stresses in higher plants. In this work we identified a 27‐kDa MIP (MIP27) in the microsomal membranes from tomato fruit using affinity purified antibodies to MIP27 from Beta vulgaris L. Sucrose gradient centrifugation analysis of microsomal membranes showed that MIP27 was associated with the plasma membrane and tonoplast fractions of tomato fruit. MIP27 aggregated to a 45‐kDa protein upon SDS‐polyacrylamide gel electrophoresis in the absence of dithiothreitol, a property characteristic of MIP proteins. MIP27 was degraded to a 25‐kDa protein as tomato fruit progressed through the various stages of ripeness. The proteolytic degradation of MIP27 to the 25‐kDa protein was also observed when microsomal membranes were treated with Pronase E. Treatment of microsomal membranes with thermolysin plus digitonin resulted in the complete degradation of MIP27. MIP27 was insensitive to treatments with trypsin and carboxypeptidase Y. The proteolytic degradation of MIP27 may play a role in the structural integrity and textural properties of tomato fruit during ripening. |
