Inhibition of proteasome activity by the TED4 protein in extracellular space: A novel mechanism for protection of living cells from injury caused by dying cells

In maturation process of tracheary element (TE) differentiation, many hydro lases are activated to execute programmed cell death of TEs, Such hydrolase s are released from maturing TEs into extracellular space. The release of h ydrolases should be harmful to surrounding cells, The TED4 protein, a tenta tive plant non-specific lipid transfer protein that is expressed preferenti ally in TE-induced culture of zinnia (Zinnia elegans L.), is secreted into the apoplastic space prior to and associated with morphological changes of TEs, Our studies on the interrelationship between the TED4 protein and prot eolytic activities using an in vitro TE differentiation system of zinnia re vealed the following facts. (1) Active proteasome is released into medium a t maturation stage of TE differentiation. (2) The TED4 protein forms a comp lex with proteasome in culture medium. (3) The TED4 protein inhibits protea some activity in the medium and crude extracts of zinnia cells. (4) The dep letion of the TED I protein from culture medium results in an increase in m ortality of other living cells, These results strongly suggest that the sec reted TED4 protein acts as an inhibitor of proteasome to protect other cell s from undesirable injury due to proteolytic activities exudated from dying TEs.