Limited proteolysis of enzyme inhibitor proteins during seed desiccation in Pisum

A number of isoforms of trypsin/chymotrypsin enzyme inhibitor proteins (TI) accumulate during seed development in Pisum (pea). The appearance of TI isoforms is not synchronous, with two appearing at earlier stages of development than other isoforms. The later isoforms accumulate preferentially in the axes of the maturing seed and during the desiccation phase of seed development, when TI activities increase in both axes and cotyledons, and are associated with a carboxy-terminal proteolytic event that increases the affinity of the primary mature proteins for target enzymes. Higher TI activities in axes from mature seed are likely to reflect both a higher relative level of post-translational processing and a different temporal pattern of TI mRNA accumulation. The enzymes implicated in the processing event may be those involved in cleavage of several seed proteins, that occurs within protein bodies. Other inhibitor genes that are expressed in vegetative organs encode proteins with divergent carboxy-terminal ends that are unlikely to be processed in the same way as the major seed inhibitors.