Limits...
Vertebrate isoforms of actin capping protein beta have distinct functions In vivo.

Hart MC, Cooper JA - J. Cell Biol. (1999)

Bottom Line: The beta2 did not localize to the Z-line.Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform.CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.

ABSTRACT
Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

Show MeSH
Immunofluorescence staining of CPβ1 and CPβ2 isoforms and corresponding DIC images in TG-wtβ1 (line 3), TG-wtβ2 (line 2), and TG-β1L262R (line 3) hearts. In TG-wtβ1 myocardium, CPβ1 localized to the Z-lines, in a pattern that appeared slightly wider than wild-type. CPβ1 staining intensity was not increased at the intercalated disc (arrow), relative to Z lines. CPβ2 localized to cell–cell junctions, the cell cortex, and in a punctate pattern. In TG-wtβ2 myocardium, CPβ1 localized to incomplete Z-lines; CPβ2 localized to the cell cortex and in a punctate pattern. In TG-β1L262R hearts, CPβ1L-262R protein localized to wavy, fragmented Z-lines (arrows) and in a punctate pattern. Bar, 5 μm.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2168092&req=5

Figure 9: Immunofluorescence staining of CPβ1 and CPβ2 isoforms and corresponding DIC images in TG-wtβ1 (line 3), TG-wtβ2 (line 2), and TG-β1L262R (line 3) hearts. In TG-wtβ1 myocardium, CPβ1 localized to the Z-lines, in a pattern that appeared slightly wider than wild-type. CPβ1 staining intensity was not increased at the intercalated disc (arrow), relative to Z lines. CPβ2 localized to cell–cell junctions, the cell cortex, and in a punctate pattern. In TG-wtβ2 myocardium, CPβ1 localized to incomplete Z-lines; CPβ2 localized to the cell cortex and in a punctate pattern. In TG-β1L262R hearts, CPβ1L-262R protein localized to wavy, fragmented Z-lines (arrows) and in a punctate pattern. Bar, 5 μm.

Mentions: In TG-wtβ2 hearts, CPβ2 localized to the periphery of the myofibrils and in a punctate pattern (Fig. 9). Because the Z-lines were truncated and wavy, we used double immunofluorescence labeling with antiactin to identify the Z-line (Fig. 10). Double immunofluorescence labeling with anti-β2 revealed that CPβ2 did not localize to the Z-line in the TG-wtβ2 myocardium. This suggests that CPβ2 is unable to functionally replace β1 at the Z-line due to its inability to localize to the Z-line.


Vertebrate isoforms of actin capping protein beta have distinct functions In vivo.

Hart MC, Cooper JA - J. Cell Biol. (1999)

Immunofluorescence staining of CPβ1 and CPβ2 isoforms and corresponding DIC images in TG-wtβ1 (line 3), TG-wtβ2 (line 2), and TG-β1L262R (line 3) hearts. In TG-wtβ1 myocardium, CPβ1 localized to the Z-lines, in a pattern that appeared slightly wider than wild-type. CPβ1 staining intensity was not increased at the intercalated disc (arrow), relative to Z lines. CPβ2 localized to cell–cell junctions, the cell cortex, and in a punctate pattern. In TG-wtβ2 myocardium, CPβ1 localized to incomplete Z-lines; CPβ2 localized to the cell cortex and in a punctate pattern. In TG-β1L262R hearts, CPβ1L-262R protein localized to wavy, fragmented Z-lines (arrows) and in a punctate pattern. Bar, 5 μm.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2168092&req=5

Figure 9: Immunofluorescence staining of CPβ1 and CPβ2 isoforms and corresponding DIC images in TG-wtβ1 (line 3), TG-wtβ2 (line 2), and TG-β1L262R (line 3) hearts. In TG-wtβ1 myocardium, CPβ1 localized to the Z-lines, in a pattern that appeared slightly wider than wild-type. CPβ1 staining intensity was not increased at the intercalated disc (arrow), relative to Z lines. CPβ2 localized to cell–cell junctions, the cell cortex, and in a punctate pattern. In TG-wtβ2 myocardium, CPβ1 localized to incomplete Z-lines; CPβ2 localized to the cell cortex and in a punctate pattern. In TG-β1L262R hearts, CPβ1L-262R protein localized to wavy, fragmented Z-lines (arrows) and in a punctate pattern. Bar, 5 μm.
Mentions: In TG-wtβ2 hearts, CPβ2 localized to the periphery of the myofibrils and in a punctate pattern (Fig. 9). Because the Z-lines were truncated and wavy, we used double immunofluorescence labeling with antiactin to identify the Z-line (Fig. 10). Double immunofluorescence labeling with anti-β2 revealed that CPβ2 did not localize to the Z-line in the TG-wtβ2 myocardium. This suggests that CPβ2 is unable to functionally replace β1 at the Z-line due to its inability to localize to the Z-line.

Bottom Line: The beta2 did not localize to the Z-line.Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform.CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.

ABSTRACT
Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

Show MeSH