PHOSPHORYLATION OF A 62 KDA PORCINE ALPHA-INTERNEXIN, A NEWLY IDENTIFIED INTERMEDIATE FILAMENT PROTEIN

A 62 kd protein was purified from the Triton-insoluble fraction of porcine brain white matter. This protein formed 10 nm filaments, in vitro. The phosphorylation of the 62 kd protien by cAMP-dependent protein kinase caused electrophoretic mobility to shift to 66 kd on SDS-PAGE and a complete loss of the filament forming ability ensued. Amino acid sequences of four peptide fragments obtained from the 62 kd protein by lysylendopeptidase were identical with that of 66 kd rat brain α-internexin. Amino acid analyses of the phosphopeptide fragment derived from phosphorylated porcine α-internexin revealed that the phosphorylation sites by cAMP-dependent protein kinase located in the amino-terminal head domain of this protein. These results strongly suggest that α-internexin polymerizes into 10 nm filaments in vitro and that phosphorylation of the amino-terminal domain of α-internexin controls its polymerizability. © 1993 Academic Press, Inc.