acción de la cápsula de proteína/funcionamiento de la proteína encapsulada
Capping protein (CP) is heterodimeric actin-binding protein found in all eukaryotic cells (reviewed in (1)). In vitro, CP binds tightly to the barbed ends of actin filaments, nucleates polymerization of actin and is inhibited by PIP2. In vivo, CP is important for actin assembly, based on studies in a variety of cell types and organisms.
Synonymous names: CapZ - CP of skeletal muscle, found at the Z line (2). Cap32/34 - CP of Dictyostelium (3). Homologous proteins: Homologues of CP are found in all eukaryotes. No other proteins share substantial sequence similarity with CP.
CP is an a/b heterodimer with subunits of Mr 32-36 kD (a) and 28-32 kD (b). CP has been found in all eukaryotes examined, including yeast (4), protozoa (5), nematodes (6), Drosophila (7), birds, and mammals (8, 9, 10). CP has been found in all cells and tissues of vertebrates (9). CP homologues show functional conservation, in that nematode and Drosophila CP can rescue yeast mutants (7, 11).
Lower organisms, yeast through Drosophila, have single genes and isoforms for each of the a and b subunits. Vertebrates have multiple isoforms, and isoform sequences are conserved from birds to mammals.
In vertebrates, the a subunit is encoded by two expressed genes, a1 and a2 (12). Mice have a third a subunit, a3, encoded by another gene, which is expressed specifically in testicular germ cells (13). Whether other vertebrates have the a3 isoform is not known. Most cells and tissues have a mixture of the a1 and a2 isoforms, at varying ratios. Endothelial cells have only a1 and erythrocytes have only a2 (12). In vitro, CP with the a1 subunit binds actin 4-fold more tightly than CP with the a2 subunit (14).
In vertebrates, the b subunit is encoded by one gene, which is alternatively spliced to produce two isoforms that differ only in a short region at the C-terminus (8). This region is necessary for actin binding (15), but CP b1 and CP b2 bind actin with similar affinity and kinetics (16). The b1 isoform is expressed in muscle tissues and located at Z lines in striated muscle. The b2 isoform is expressed in all cells, and is the predominant or exclusive b isoform in non-muscle cells. In striated muscle, b2 is located at cell-cell junctions, such as intercalated disks in heart. A novel CP b isoform, called b3, has been described in bovine sperm. This isoform includes a conventional vertebrate CP b2 sequence with an N-terminal extension of ~30 amino acids (Z85980) (16a).
Regulatory mechanisms. CP is inhibited by polyphosphoinositides. PIP2 rapidly and efficiently removes CP from barbed ends of actin filaments, which is a possible mechanism to account for cases where free barbed ends rapidly appear in vivo (16). No post-translational modifications of CP have been found. Hsc70 can be associated with CP, presumably as a chaperone, but does not otherwise affect the activity of CP (3, 16). S100 also binds CP but does not affect its activity (16, 17).