Expression of wild-type and mutant rat liver CTP: phosphocholine cytidylyltransferase in a cytidylyltransferase-deficient Chinese hamster ovary cell line

The strain 58 Chinese hamster ovary (CHO) mutant defective in CTP:phosphocholine cytidylyltransferase was characterized as an expression system for exogenous cytidylyltransferase. Strain 58 cells express less than 5% of the wild-type level of cytidylyltransferase protein at the permissive temperature even though the steady-state messenger RNA levels were found to be similar to those in the parental CHO-K1 cell line. A point mutation from arginine to histidine at amino acid 140 was identified in the strain 58 protein. Rat liver cytidylyltransferase was stably expressed in strain 58 cells and shown to be active, targeted to the nucleus, phosphorylated, and activated by methylethanolamine supplementation or phospholipase C treatment. Thus, the mechanisms by which cytidylyltransferase is processed and regulated in CHO-K1 cells are intact in strain 58 cells. The heterologously expressed protein complemented the strain 58 defects in both temperature-sensitive growth and phosphatidylcholine biosynthesis, consistent with a single lesion in the structural gene for cytidylyltransferase being responsible for both phenomena. Overexpression of cytidylyltransferase activity at levels up to eightfold higher than those in CHO-K1 cells did not appreciably affect phosphatidylcholine metabolism. A putative casein kinase II phosphorylation site was altered by site-directed mutagenesis and expressed in the strain 58 cells. Alteration of this site did not affect expression and regulation of cytidylyltransferase activity.

Differential expression of GAP-43 mRNA in adult central cholinergic neuronal populations

The present study addresses a question of differential expression for a 'plasticity' gene within neurons identified by neurotransmitter type. A method combining immunohistochemical localization of choline acetyltransferase (ChAT) with in situ hybridization histochemistry (ISHH) in the same brain sections was used to quantitate the levels of mRNA for the growth-associated protein GAP-43 (neuromodulin) in rat central cholinergic neuronal populations. We found that many cholinergic neurons in the adult rat brain express levels of GAP-43 mRNA comparable to other brain regions noted for their expression of this plasticity gene. GAP-43 expression in cholinergic cell groups appeared to be highly heterogeneous; this was often true even for cholinergic neurons within the same brain region. A dorsal-ventral gradient in GAP-43 mRNA levels was evident in the rostral basal forebrain cholinergic groups; i.e., medial septum, nucleus basalis magnocellularis and the vertical limb of the diagonal band expressed intermediate levels, while the horizontal limb of the diagonal band and the substantia innominata expressed higher levels of GAP-43 mRNA. Cholinergic interneurons of the striatum were grouped in several populations according to mRNA levels, from very low to very high expression. Similarly, GAP-43 mRNA levels in the cholinergic neurons of the nucleus basalis magnocellularis/substantia innominata were quite variable. The expression of GAP-43 mRNA in brainstem cholinergic groups (laterodorsal tegmental and pedunculopontine nuclei) was in nearly uniform populations of somewhat lower levels. While the expression of GAP-43 mRNA in cholinergic neurons was heterogeneous, virtually every ChAT-positive neuron sampled contained GAP-43 mRNA at levels significantly over background.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of phosphorylation sites in rat liver CTP: phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CT) is an important regulatory enzyme in phosphatidylcholine biosynthesis. The enzyme exists as a soluble, inactive form that is highly phosphorylated; activation of the enzyme is accompanied by dephosphorylation and translocation to the membrane. We have used a recombinant baculovirus clone to obtain CT labeled in vivo with 32PO4. The tryptic phosphopeptide pattern of the baculovirus-expressed CT was the same as for CT expressed in mammalian cells, indicating that insect cells modify the same phosphorylation sites as do mammalian cells. 32PO4-labeled, baculovirus-expressed CT was digested with trypsin and the peptides purified by reversed-phase high performance liquid chromatography. Phosphoamino acid analysis of the complete protein as well as individual peptides revealed that only serine residues were phosphorylated. Sequence analysis of purified radioactive peptides revealed that phosphorylation of CT was confined to the carboxyl-terminal region and that all or nearly all Ser residues from Ser315 to the carboxyl terminus were labeled. Ser315, Ser319, Ser329, Ser323, Ser331, Ser343, and Ser347 all reside in potential sites for proline-directed kinases. Two other phosphorylated serine residues, Ser315 and Ser333, are found within protein kinase C consensus phosphorylation sites. Ser321, Ser322, Ser333, Ser345, Ser346, Ser350, Ser352, and Ser362 were also found to be phosphorylated. Serine362 resides within a putative casein kinase II phosphorylation site, and there are five potential sites for phosphorylation by glycogen synthase kinase 3. Identification of these sites will allow investigations that focus on the establishment of the physiological function of phosphorylation at each site.