O-phosphoserine content of intermediate filament subunits

Purified subunits of intermediate filaments obtained from a variety of tissues and cell types contain O-phosphoserine and, in some cases, smaller amounts of O-phosphothreonine. The O-phosphoserine content was estimated by reaction of performic acid oxidized subunits with methylamine in NaOH. Decamin of BHK-21 and CHO fibroblasts contained about 1 mol/mol. Avian and mammalian desmin consists of two subunits, an acidic (alpha) subunit which contained 2 mol/mol and a more basic (beta) nonphosphorylated subunit. The principal (Mr approximately 60 000) subunit of squid brain neurofilaments contained 5 mol/mol. Most mouse and bovine keratin subunits contained 3--6 mol/mol, although certain bovine subunits of higher molecular weight contained none. The O-phosphoserine contents of keratin subunits purified from the viable and stratum corneum layers were the same. The O-phosphoserine was located in non-alpha-helical regions of the subunits which presumably project out from the alpha-helical wall of the intermediate filaments. Most subunits could be partially dephosphorylated in vitro with alkaline phosphatase. It was found that the capacity of such partially dephosphorylated subunits for assembly into native-type filaments in vitro was independent of their phosphate content.