Identification of plectin as a substrate of p34(cdc2) kinase and mapping of a single phosphorylation site

Plectin is an in vitro substrate for various kinases present in cell lysates from mitotic and interphase Chinese hamster ovary cells. Sensitivity of plectin kinase activity to the inhibitor olomoucine, and two-dimensional tryptic peptide mapping of plectin phosphorylated by various kinase preparations suggested that the major plectin kinase activity in mitotic extracts is related to the cell cycle regulator kinase p34(cdc2). Bacterial expression of various truncated plectin mutant proteins comprising different domains of the molecule and their phosphorylation by purified p34(cdc2) kinase revealed that the target site of this kinase resided within plectin's C-terminal globular domain. Among the subdomains of the C-terminal region (six repeats and a short tail sequence), only repeat 6 and the tail were phosphorylated by p34(cdc2) kinase. As shown by two-dimensional phosphopeptide mapping, repeat 6, but not the tail, contained a mitosis-specific phosphorylation site targeted by p34(cdc2) kinase in intact plectin molecules. By performing site-directed mutagenesis of a potential p34(cdc2) recognition sequence motif within the repeat 6 domain, threonine 4542 was identified as the major target for the kinase. Protein kinase A, phosphorylating plectin also within repeat 6, targeted sites that were clearly different from those of p34(cdc2) kinase.