Abstract

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The cellular glycoprotein, p67 binds directly to eukaryotic initiation factor 2 (eIF2) and extracellular signal-regulated kinases 1 and 2 (ERK1/2) and regulates their levels of phosphorylation. The cDNA sequences for p67 homologs were cloned from various species such as yeast, insects, plants, mouse, rat, and human. The rat cDNA was extensively characterized by detailed mutagenesis accompanied with a series of biochemical analysis in in vitro, ex vivo, and in vivo assays. These assays identified a series of functional domains/motifs that play important roles in p67’s multi-faceted activities such as blocking phosphorylation of eIF2, ERK1/2, and possibly other unknown kinases and intra-molecular (auto-proteolysis) and intermolecular proteolysis activity. Auto-proteolysis of rat and human p67 generates several peptide fragments. The most stable fragments are p26 that covers N-terminal 1-107 amino acid residues and p52 that covers downstream 108-480 amino acid residues. The p26 segment seems to be the business end of the molecule, whose activity is regulated by the p52 segment. In the p52 segment there are at least five conserved amino acid residues – D251, D262, H331, E364, and E459. These amino acids after folding juxtapose together to create a shallow groove, which serves as the substrate-binding pocket and the nearby H231 residue serves as the catalytic site for p67’s intra- and/or inter-molecular activities to cleave peptide bonds. The target for intra-molecular proteolysis is p67 itself, whereas the inter-molecular targets are cell cycle regulatory proteins, certain cyclins and Cdks, and p21; cell growth, cytoskeleton dynamics, migration, and motility regulatory proteins, PKC, MARCKS, FAK, Cdc42, PAK1, and Akt1/2 (or PKB); eIF2-specific kinase, PERK; and possibly several other unidentified kinases and phosphor-proteins that are involved in cell signaling. By proteolytic cleaving various proteins/kinases p67 is involved in the regulation of different cellular processes.

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