A spectrin‐like protein from mouse brain membranes: Immunological and structural correlations with erythrocyte spectrin

Membrane‐associated mouse brain spectrin is a 972,000 M_r, 10.5S, (αβ)₂ tetramer containing two ∼ 240,000 M_r subunits and two ∼ 235,000 M_r subunits. Two‐dimensional [¹²⁵I]tryptic peptide mapping indicates that these subunits share only limited and equivalent overlap with the α‐ and β‐subunits of red blood cell (RBC) spectrin. Both the 220,000 M_r β‐subunit of RBC spectrin and the 235,000 M_r β‐subunit of brain spectrin are phosphorylated in the intact mouse. In vitro analysis suggests that both are phosphorylated by a cAMP‐independent protein kinase. Antibodies against pure native mouse red blood cell spectrin cross‐react with brain spectrin, and antibodies against pure brain spectrin cross‐react with both the α‐and β‐subunits of mouse RBC spectrin. Both antibodies have been utilized to localize brain spectrin within distinct cellular entities of the mouse cerebellum. Granule cell neurons of the internal granule layer and Purkinje cell neurons demonstrated intense fluorscence of the cortical cytoplasm immediately adjacent to the plasma membrane and unstained nuclei, when either RBC or brain spectrin antibodies were utilized for staining. The molecular layer of the cerebellum stained only lightly, and oligodendrocytes and astrocytes appeared to have little fluorescence. Therefore, while brain is a tissue rich in nonerythroid spectrin, the concentration of these immunoreactive analogues is quite variable within distinct cellular entities of the cerebellum.