Differential localization and sequence analysis of capping protein beta-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 beta subunit of chicken capping protein, the beta 2 isoform, which arises by alternative splicing. The chicken beta 1 isoform and the beta 2 isoform are identical in their amino acid sequence except for a short region at the COOH terminus; this region of the beta subunit has been implicated in binding actin. Human and mouse cDNAs of the beta 1 and beta 2 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 beta subunit COOH-termini to those of lower eukaryotes shows no similarities. The beta 2 isoform is the predominant isoform of nonmuscle tissues and the beta 1 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 beta 2 isoform also is a component of dynactin complex from brain, which contains the actin-related protein Arp1. Both beta-subunit isoforms are expressed in cardiac muscle but they have non-overlapping subcellular distributions. The beta 1 isoform is at Z-discs of myofibrils, and the beta 2 isoform is enriched at intercalated discs; in cardiac myocytes grown in culture, the beta 2 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.
Schafer
D A
DA
Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110.
Korshunova
Y O
YO
Schroer
T A
TA
Cooper
J A
JA
D051339:Actin Depolymerizing Factors; D000199:Actins / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001923:Brain Chemistry; D002478:Cells, Cultured; D002645:Chickens; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D051345:Destrin; D000072159:Dynactin Complex; D005895:Gizzard, Avian / Q000737:chemistry; D006801:Humans; D007365:Intercellular Junctions / Q000737:chemistry; D008099:Liver / Q000737:chemistry; D051379:Mice; D008840:Microfilament Proteins / Q000032:analysis / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008868:Microtubule Proteins / Q000737:chemistry; D008869:Microtubule-Associated Proteins; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000737:chemistry; D009206:Myocardium / Q000737:chemistry / Q000166:cytology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D017421:Sequence Analysis
D051339:Actin Depolymerizing Factors; D000199:Actins; D018076:DNA, Complementary; D051345:Destrin; D000072159:Dynactin Complex; D008840:Microfilament Proteins; D008868:Microtubule Proteins; D008869:Microtubule-Associated Proteins; D012333:RNA, Messenger