Differential localization and sequence analysis of capping protein β-subunit isoforms of vertebrates

Capping protein nucleates the assembly of actin filaments and stabilizes actin filaments by binding to their barbed ends. We describe here a novel isoform of the β subunit of chicken capping protein, the β2 isoform, which arises by alternative splicing. The chicken β1 isoform and the β2 isoform are identical in their amino acid sequence except for a short region at the COOH terminus; this region of the β subunit has been implicated in binding actin. Human and mouse cDNAs of the β1 and β2 isoforms also were isolated and among these vertebrates, the COOH-terminal region of each isoform is highly conserved. In contrast, comparison of the sequences of the vertebrate β subunit COOH-termini to those of lower eukaryotes shows no similarities. The β2 isoform is the predominant isoform of non-muscle tissues and the β1 isoform, which was first characterized in studies of capping protein from chicken muscle, is the predominant isoform of muscle tissues, as shown by immunoblots probed with isoform-specific antibodies and by RNAse protection analysis of mRNAs. The β2 isoform also is a component of dynactin complex from brain, which contains the actin-related protein Arp1. Both β-subunit isoforms are expressed in cardiac muscle but they have non-overlapping subcellular distributions. The β1 isoform is at Z-discs of myofibrils, and the β2 isoform is enriched at intercalated discs; in cardiac myocytes grown in culture, the β2 isoform also is a component of cell-cell junctions and at sites where myofibrils contact the sarcolemma. The biochemical basis for the differential distribution of capping protein isoforms is likely due to interaction with specific proteins at Z-discs and cell-cell junctions, or to preferential association with different actin isoforms. Thus, vertebrates have developed isoforms of capping protein that associate with distinct actin-filament arrays.