ABSTRACT
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.
Subject(s)
Cytoskeleton/analysis , Phosphoserine/analysis , Serine/analogs & derivatives , Animals , Cattle , Cricetinae , Cricetulus , Cytoskeleton/physiology , Decapodiformes , Desmin , Female , Keratins/analysis , Mesocricetus , Methods , Methylamines , Mice , Mice, Inbred BALB C , Muscle Proteins/analysis , Phosphorylation , Phosphothreonine/analysis , Tissue Distribution , Turkeys , VimentinABSTRACT
Steinert [Biochem. J. (1975) 149, 39-48] reported that the alpha-keratin polypeptides (the subunits of the intracellular keratin filaments) of bovine hoof and snout epidermis are the same. We now demonstrate that this is not so: in addition to the seven polypeptides previously identified in hoof epidermis, snout epidermis also contains at least three other polypeptides of higher molecular weight. These unique polypeptides were isolated, purified and characterized. They are chemically and structurally very similar to the other polypeptides of bovine epidermis and readily polymerize in vitro with them to form native-type epidermal keratin filaments.
