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Characterization of Infant Rat Cerebral Cortical Membrane Proteins Phosphorylated In Vivo: Identification of the ACTH‐Sensitive Phosphoprotein B‐50

Abstract: This study on the phosphorylation in vivo of membrane proteins in cerebral cortices of infant rats reports the identification of the adrenocorticotropin (ACTH)‐sensitive phosphoprotein B‐50 as one of the substrate proteins that are rapidly phosphorylated in vivo following intracisternal administration of 2 mCi [32P]orthophosphate. Rats were sacrificed 30 min after isotope injection. A fraction enriched in membranes, designated neural membranes (NM), was isolated from the cerebral cortices according to the procedure used for preparation of synaptic plasma membranes (SPM) from adult brain. This NM fraction was characterized by electron microscopy. The proteins of NM were analyzed by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis and autoradiography. Numerous protein bands of NM in infant rat brain were phosphorylated in vivo. Attention was focussed on the 32P‐labeled protein bands in the molecular weight range of 47K‐67K. In this region one phosphoprotein band (MW 48K) was more highly labeled than the other bands. The electrophoretic behavior of three of these labeled bands, designated a, c, and e (MW 48K, 55K, and 62K, respectively) was compared with that of protein bands that were phosphorylated in vitro in cerebral membranes isolated from noninjected infant rats. The effects of ACTH1‐24 and cyclic AMP in the in vitro system were also studied to probe for the presence of specific membrane proteins known to be sensitive to these modulators. On incubation of NM with [γ‐32P]ATP in the presence and absence of ACTH1‐24in vitro, phosphorylation of a 48K protein band was inhibited in a dose‐dependent fashion by the neuropeptide. Two‐dimensional electrophoretic separation of NM proteins labeled in vivo indicated that the 48K band had an isoelectric point of 4.5, identical to that of the ACTH‐sensitive B‐50 protein previously identified. Cyclic AMP stimulated phosphorylation in vitro of two protein bands (MW 55K and 59K) in NM preparations. This result indicates that the in vivo labeled band c may correspond to the cyclic AMP‐sensitive 55K protein, whereas phosphoprotein band e, labeled in vivo, appears to be different from the cyclic AMP‐sensitive 59K protein band. These observations indicate that neural membranes isolated from infant rat cerebral cortices contain a variety of proteins that can be phosphorylated in vivo. Several of these, for example, the 48K protein band, have the properties of synaptic plasma membrane proteins of adult rat brain that have been characterized by their sensitivity to neuromodulators in endogenous phosphorylating systems in vitro.