Ser‐59 is the major phosphorylation site in αB‐crystallin accumulated in the brains of patients with Alexander's disease

The phosphorylation state of αB‐crystallin accumulated in the brains of two patients with Alexander's disease (one infantile and one juvenile type) was determined by means of SDS‐PAGE or isoelectric focusing of soluble and insoluble fractions of brain extracts and subsequent western blot analysis with specific antibodies against αB‐crystallin and each of three phosphorylated serine residues. The level of mammalian small heat shock protein of 25–28 kDa (Hsp27) in the same fraction was also estimated by western blot analysis. The majority of αB‐crystallin was detected in the insoluble fraction of brain homogenates and phosphorylation was preferentially observed at Ser‐59 in both cases. A significant level of phosphorylation at Ser‐45 but not Ser‐19 was also detected. Hsp27 was found at considerable levels in the insoluble fractions. αB‐crystallin and phosphorylated forms were detected in the cerebrospinal fluid of patient with the juvenile type. αB‐crystallin and phosphorylated forms were also detectable at considerable levels in the insoluble fraction of brain homogenates from patients with Alzheimer's disease and aged controls. The phosphorylation site was mostly at Ser‐59 in all cases. Immunohistochemically, αB‐crystallin was stained in Rosenthal fibers in brains of patients with Alexander's disease and their peripheral portions were immunostained with antibodies recognizing phosphorylated Ser‐59. These results indicate that the major phosphorylation site in αB‐crystallin in brains of patients with Alexander's disease or Alzheimer's disease as well as in aged controls is Ser‐59.

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