Mapping of the Mouse Actin Capping Protein Beta Subunit Gene

We isolated genomic clones corresponding to the β subunit of mouse CP and identified its chromosomal location by interspecies backcross mapping.The CPβ gene (Cappb1) mapped to Chromosome 4 between Cdc42 and D4Mit312. Three mouse mutations, snubnose, curly tail, and cribriform degeneration, map in the vicinity of the β gene.Capping protein (CP) is a ubiquitous actin binding protein that regulates actin assembly and cell motility. CP is a heterodimer composed of α and β subunits, each approximately 30 kD. Lower organisms, including Saccharomyces cerevisiae, Caenorhabditis elegans and Drosophila melanogaster, have one gene and one isoform for each of the CPα and β subunits [1,2,3]. Vertebrates contain three α subunit isoforms encoded by three different genes and three β subunit isoforms (β1, β2, β3) produced from one gene by alternative splicing [4,5,6].To confirm the genomic complexity of the β subunit of mouse CP, Southern blots of mouse genomic DNA were cleaved with several restriction enzymes and independently probed with mouse β1 and β2 cDNAs. The β1 and β2 probes each hybridized to one identical band. To obtain genomic clones that correspond to this pattern, a 129SV genomic library in the lambda FIXII vector (Stratagene) was screened using the complete β1 cDNA as probe. Positive plaques were purified according to standard procedures [7] and the phage DNA isolated using DEAE-cellulose [8]. Comparison of the hybridization pattern for the genomic clones with the mouse genomic DNA revealed that the patterns were identical, confirming the presence of a single β gene in the mouse genome.In chicken, the β1 and β2 isoforms have been described as the differentially spliced products of a single gene, with the β1 cDNA containing a 113 bp exon that is absent in the β2 cDNA [4]. To determine if the murine β2 isoform is spliced in a manner identical to that of chicken, the sequence of the β2 cDNA was compared to the corresponding genomic region. Sequence comparison revealed th