THE SITES AT WHICH BRAIN MICROTUBULE-ASSOCIATED PROTEIN-2 IS PHOSPHORYLATED INVIVO DIFFER FROM THOSE ACCESSIBLE TO CAMP-DEPENDENT KINASE INVITRO

  • 1 January 1985
    • journal article
    • research article
    • Vol. 260  (7) , 4364-4370
Abstract
The most conspicuous brain microtubule-associated protein, MAP-2, contained 8-10 mol of covalently bound phosphate/mol, as isolated. The MAP-2-associated cAMP-dependent protein kinase can add 10-12 more phosphates using cycled microtubule preparations, but it does not catalyze exchange between ATP and the pre-existing protein phosphate. The phosphates that turn over in vivo, after intracerebral injection of 32Pi, are primarily in the projection domain of MAP-2, whereas the sites phosphorylated in vitro are more concentrated in the binding domain. Phosphoserine and phosphothereonine were recovered in a 6:1 ratio from partial acid hydrolysates of MAP-2, phosphorylated either in vivo or in vitro. A protein phosphatase, purified from brain, released residues from in vitro sites in both domains. The enzyme did not release appreciable phosphate that had turned over in vivo; similar specificity was shown by 3 other purified protein phosphatases: calcineurin (also from brain) and smooth muscle phosphatases I and II. Thus, even in the projection domain, different sites may be involved.