Mechanisms for the genesis of aldolase tetramers in cell-free protein synthesizing systems and in vivo

Localization of aldolase C mRNA in brain cells

The expression of aldolase C and aldolase A mRNA was assessed by Northern blot hybridization using RNAs purified from cultured rat and mouse brain neurons and astroglial cells. Neurons were found to contain about 4-fold more aldolase C mRNA and about twice as much aldolase A mRNA than astroglia. Analysis of the cellular localization of aldolase C mRNA by in situ hybridization to brain slices showed a predominantly neuronal labeling with an irregular distribution. A strong signal was observed in Purkinje cell somata and a weaker signal in subpopulations of neurons in cerebral cortex, striatum, hippocampus, hypothalamic nuclei and primary olfactory cortex.

Two different aldolase A mRNA species in rat tissues

Double-stranded DNA was synthesized with reverse transcriptase from size-fractionated poly(A)-containing RNA from rat ascites hepatoma cells. The cDNA was introduced into Escherichia coli HB101 using pBR322 DNA as a cloning vector. Several plasmids containing aldolase A cDNA were identified by colony hybridization with 32P-labeled cDNA prepared from immunologically purified aldolase A mRNA. The partial amino acid sequence of the cDNA sequence was determined, and found to coincide with that of rabbit aldolase A. Using aldolase A cDNA as a hybridization probe, the aldolase A mRNA concentrations in various rat tissues were analysed, and two aldolase A mRNA species differing in nucleotide length were found; the smaller mRNA (about 1550 nucleotides) in muscle, and the larger one (about 1650 nucleotides) in brain and hepatoma cells.

Expression of aldolase isozyme mRNAs in fetal rat liver

The regulation of aldolase isozyme expression during development was studied by measuring the concentrations of mRNAs coding for aldolase A and B subunits in fetal and adult rat liver. Poly(A)-containing RNAs were extracted from livers at various stages of development of fetal rats, and the aldolase A and B subunits in the in vitro translation products of these RNAs were analyzed immunologically. The content of aldolase B mRNA in 14-day fetal liver, measured quantitatively as translational activity, was somewhat smaller than that of aldolase A mRNA; immunologically precipitable aldolase B and A amounted to 0.06% and 0.25% respectively, of the total products. Similar experiments using RNAs from fetuses at later stages, however, showed that aldolase B mRNA increased during development, whereas aldolase A mRNA decreased. In newborn rat liver, aldolase B constituted 0.56% of the total translation products of mRNA, but there was little detectable aldolase A (0.03%). The changes of aldolase mRNA levels were analyzed further by northern blot and dot-blot hybridization experiments using cloned aldolase A and B cDNAs. The content of aldolase B mRNA increased in the fetal stage, and that in newborn rat liver was about 12 times that in 14-day fetal liver. In contrast, the aldolase A mRNA content decreased during gestation and that in newborn rat liver was about one-eighth of that in 14-day fetal liver. These observations suggest that the switch of aldolase isozyme expression in fetal liver is controlled by the levels of the respective mRNAs.

Cellular localization of aldolase C subunits in human brain

An antiserum specific for the brain-type aldolase C subunit has been used to investigate the cellular localization of this protein in human brain. Immunoperoxidase labeling at the light microscope level showed heavy staining of Purkinje cells in the cerebellum and of astrocytes in the cerebrum. Faint staining of occasional large neurons in the gray matter of the cerebral cortex was also seen.

Studies on the structure of aldolase A from chicken muscle

S-Carboxymethylated chicken muscle aldolase was treated with cyanogen bromide to cleave the 4 methionyl bonds per subunit. Five homogeneous fractions were obtained designated fragments I-V. Fragment I was derived from the N-terminus and fragment II from the C-terminus of the enzyme. Reduction of the enzyme with NaB3H4 in the presence of dihydroxyacetone phosphate decreases the enzymatic activity by 90%. Fragment III contained the Schiff base-forming lysine residue since more than 83% of the radioactivity introduced by NaB3H4 reduction of aldolase-dihydroxyacetone phosphate was found in this fraction. A tryptic peptide of 27 amino acid residues containing the substrate-binding site was isolated. The gross molecular structure of aldolase A from chicken muscle indicates a high degree of homology with mammalian muscle aldolases.