Acceptors for cyclic AMP-dependent and calcium ion-dependent protein kinases in rat brain cytosol fractions: a comparison of occluded (synaptosomal) cytosol with non-occluded cytosol

Dephosphin, a 96,000 Da substrate of protein kinase C in synaptosomal cytosol, is phosphorylated in intact synaptosomes

A 96,000 dalton phosphoprotein, called dephosphin, is phosphorylated in intact synaptosomes from rat brain and is rapidly dephosphorylated upon depolarisation-dependent calcium entry. A 96,000 dalton phosphoprotein is also a substrate of protein kinase C in synaptosomal cytosol, and the aim of the study was to determine whether the two proteins may be the same. Dephosphin in intact synaptosomes and the 96,000 dalton protein kinase C substrate comigrated on polyacrylamide gels. Both phosphoproteins had identical phosphopeptide maps after digestion with V8 protease. Both phosphoproteins ran on isoelectric focussing gels with a pI of 6.3-6.7 and focussed as a series of 5-6 spots. Both proteins were phosphorylated exclusively on serine. Both proteins could be resolved into a doublet on longer polyacrylamide gels. The two subunits were of 96 and 93 kDa in both phosphorylation conditions and had dissimilar phosphopeptide maps. However, phosphopeptide maps of either the 96 or 93 kDa subunits were identical in intact synaptosomes compared with synaptosomal cytosol. These results show that a phosphoprotein phosphorylated in intact synaptosomes and a 96,000 dalton protein kinase C substrate from rat brain synaptosomal cytosol are the same, and raise the possibility that protein kinase C is the protein kinase responsible for dephosphin phosphorylation in intact synaptosomes.

Developmental changes in protein phosphorylation in chicken forebrain. I. cAMP-stimulated phosphorylation

The net level of cyclic AMP-stimulated protein phosphorylation was investigated in cytosolic and membrane fractions from chicken forebrain between embryonic day 13 (E13) and 52 days post-hatching. Throughout this period the majority of the net level of cAMP-stimulated phosphorylation of endogenous proteins was in the cytosolic fractions. Between day -8 (E13) and adult, the net level of cAMP-stimulated phosphorylation of endogenous proteins in the cytosol (S3) and crude synaptic plasma membrane (P2-M) fractions fell by 3 and 4 fold, respectively, when expressed per mg protein and rose by 5 and 10 fold, respectively, when expressed per fraction. The changes in specific activity were completed by 6-15 days post-hatching. The occluded cytosol (P2-S) fraction showed little change in the net level of cAMP-stimulated phosphorylation of endogenous proteins per mg protein. Major changes in phosphoprotein patterns involving both decreases and increases in phosphorylation occurred in all fractions from day -8 (E13) to day 6 post-hatch; thereafter the phosphoprotein bands and their relative intensities were unchanged. Three bands (P90 in S3; P41 and P31 in P2-M) contained major cAMP-stimulated phosphoproteins in embryonic brain but were absent after hatching. When cAMP-stimulated phosphorylation activity was measured in S3 and P-2M using an exogenous peptide substrate (Kemptide) there was no change in kinase activity per mg protein between day -8 (E13) and 30 days post-hatch. This suggests that the decrease in the net level of cAMP stimulated phosphorylation of endogenous proteins was due to the decrease in levels of endogenous phosphoproteins rather than protein kinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in protein phosphorylation in chicken forebrain. II. Calmodulin stimulated phosphorylation

The development of calmodulin stimulated protein phosphorylation, with particular reference to calmodulin-stimulated protein kinase II (CMK II), was investigated in 3 subcellular fractions of chicken forebrain: cytosol (S3), crude synaptic plasma membranes (P2-M) and occluded cytosol (P2-S). Changes in the level of calmodulin-stimulated phosphorylation of endogenous proteins occurred over a protracted time course and were not complete until after day 52 post-hatching. By day 15 post-hatching, calmodulin-stimulated phosphoproteins characteristic of embryonic fractions had all disappeared and those characteristic of adult tissue were present but not necessarily at their mature levels. The levels of CMK II were estimated from the autophosphorylation of the alpha-subunit which was the only phosphoprotein present at 53,000 Da in the 3 fractions. Overall, calmodulin-stimulated phosphorylation and CMK II levels were low in embryonic brain and high in adult brain but two specific changes in CMK II were observed during development: (1) although CMK II concentrations increased in both membrane and cytosolic fractions until day 23 the kinase was predominantly cytoplasmic (approximately 75%) until day 23, after which it became increasingly membrane bound so that by day 52 post-hatching the majority of CMK II was present in the synaptic membrane fraction, and (2) the relative concentrations of the alpha- and beta-subunits changed from an alpha:beta-value of approximately 1:1 in the 19 day embryo to approximately 1:2 by 15 days post-hatch after which no further change was seen. The occurrence of major changes in the calmodulin stimulated protein phosphorylation system for up to 6-8 weeks after synapse formation is completed in the forebrain, provides further support for the existence of a synapse maturation phase of neuronal differentiation which is distinct from synapse formation. This phase involves only a specific subset of the developmental changes occurring in the calmodulin-stimulated phosphorylation system.

Demonstration by phase-partitioning in Triton X-114 solutions that phosphoprotein B-50 (F-1) from rat brain is an integral membrane protein

The Triton X-114 phase separation technique was employed to fractionate phosphoproteins present in membrane fragments from rat brain. Membranes were labelled with [gamma-32P]ATP in media containing Ca2+, Ca2+ plus calmodulin or cyclic AMP, and then treated with Triton X-114. Phosphoproteins recovered in the detergent-insoluble fraction, aqueous and detergent phases were detected by SDS-polyacrylamide gel electrophoresis and autoradiography. Of the proteins solubilised by the detergent, a known substrate of protein kinase C, the B-50 phosphoprotein (45 kD; also known as F-1), partitioned quantitatively into the detergent-rich phase, making it very probable that this phosphoprotein is an integral membrane protein. The detergent-rich phase also contained an 80 kD phosphoprotein, which probably corresponds to the widespread acidic 87 kD substrate of protein kinase C.