p67 has auto-proteolytic activity that generates several peptides including p26 segment containing the N-terminal 1-107 amino acid residues and p52 segment, which contains downstream 108-480 amino acid residues. p52 segment has the substrate-binding pocket and the catalytic site that regulates the cellular concentration of p26 segment, which modulates the level of phosphorylation of several kinases and regulatory proteins. Excessive amount of p26 segment could be lethal to mammalian cells. We therefore asked how mammalian cells maintain the cellular concentration of p26 segment especially during cell cycle. We found that p26 segment is in fact excreted from the cytoplasm and the unique domains such as lysine residue-rich domains I & II, and the acidic residue-rich domain do not interfere in this secretion process. We also found that p26 segment is excreted from the cytoplasm at different time of cell cycle while keeping a steady level inside the cytoplasm. Together, our data suggest that p26 segment is secreted from the cytoplasm to extracellular space possibly during physiological and pathological conditions.
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