Appearance of CAD activity, the rate-limiting enzyme for pyrimidine biosynthesis, as B cells progress into and through the G1 stage of the cell cycle.

CAD is a multifunctional protein which mediates the first three enzymatic steps of pyrimidine biosynthesis. Previous studies have implicated CAD as a cell cycle regulated protein. In the present paper CAD activity is studied as polyclonally stimulated, murine B cells progress through the early stages of the cell cycle. CAD activity is seen to increase in a biphasic manner. The initial increase in activity occurs prior to or as the cells increase CAD mRNA suggesting that post-translational modification of preformed enzyme may account for at least a portion of this initial enhancement. Increases in CAD mRNA occur by 12 hr poststimulation and precede the second, more dramatic increase in B cell CAD activity. Preliminary experiments failed to provide support of a role for IL-4 in regulating the expression of CAD as B cells progress into G1. CAD enzymatic activity does represent, however, a marker for early B cell cycle progression.

Carbohydrate-binding protein 35: molecular cloning and expression of a recombinant polypeptide with lectin activity in Escherichia coli.

Affinity-purified antibodies directed against carbohydrate-binding protein 35 (CBP35), a galactose-specific lectin, were used to screen a lambda gt 11 expression library derived from mRNA of 3T3 fibroblasts. This screening yielded several putative clones containing cDNA for CBP35, one of which was characterized in terms of its expression of a fusion protein containing beta-galactosidase and CBP35 sequences. Limited proteolysis of lysates containing the fusion protein, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with anti-CBP35, yielded a peptide mapping pattern comparable to that obtained from parallel treatment of authentic CBP35. Such a limited proteolysis followed by affinity chromatography on a Sepharose column coupled with galactose also yielded a 30-kDa polypeptide that exhibited carbohydrate-binding activity. This polypeptide can be immunoblotted with anti-CBP35, but not with antibodies directed against beta-galactosidase. These results indicate that we have identified a cDNA clone for CBP35 that yields a recombinant polypeptide with lectin activity produced in Escherichia coli. Using this cDNA clone as a probe, Northern-blot analysis showed an increased expression of the CBP35 gene when quiescent 3T3 cells were activated by the addition of serum growth factors.