Abstract A 62 kd protein was purified from the Triton-insoluble fraction of porcine brain white matter. This protein formed 10nm filaments, in vitro. The phosphorylation of the 62 kd protein 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 a 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 10nm filaments in vitro and that phosphorylation of the amino-terminal domain of α-internexin controls its polymerizability.