Da. Schafer et al., LOCALIZATION OF CAPZ DURING MYOFIBRILLOGENESIS IN CULTURED CHICKEN MUSCLE, Cell motility and the cytoskeleton, 25(4), 1993, pp. 317-335
Actin filaments undergo dramatic changes in their organization during
myofibrillogenesis. In mature skeletal muscle, both CapZ and the barbe
d end of the actin filaments are located at Z-discs. In vitro, CapZ bi
nds the barbed end of actin filaments and prevents actin subunit addit
ion and loss; CapZ also nucleates actin polymerization in vitro. Taken
together, these properties suggest that CapZ may function to organize
actin filaments during myofibrillogenesis. We report here that the am
ount of CapZ in myofibrils from adult chicken pectoral muscle is suffi
cient to ''cap'' each actin filament of the sacromere. Double immunofl
uorescence microscopy of skeletal muscle cells in culture was used to
determine the spatial and temporal distributions of CapZ relative to a
ctin, alpha-actinin, titin, and myosin during myofibrillogenesis. Of p
articular interest was the assembly of CapZ at nascent Z-discs in rela
tion to the organization of actin filaments in nascent myofibrils. In
myoblasts and young myotubes, CapZ was diffusely distributed in the cy
toplasm. As myotubes matured, CapZ was initially observed in a uniform
distribution along non-striated actin filaments called stress fiber-l
ike structures (SFLS). CapZ was observed in a periodic pattern charact
eristic of mature Z-discs along the SFLS prior to the appearance of a
striated staining pattern for actin. In older myotubes, when actin was
observed in a pattern characteristic of I-bands, CapZ was distributed
in a periodic pattern characteristic of mature Z-discs. The finding t
hat CapZ was assembled at nascent Z-discs before actin was observed in
a striated pattern is consistent with the hypothesis that CapZ direct
s the location and polarity of actin filaments during 1-band formation
in skeletal muscle cells. The assembly of CapZ at nascent Z-disc stru
ctures also was observed relative to the assembly of sarcomeric alpha-
actinin, titin, and thick filaments. Titin and myosin were observed in
structures having the organization of mature sarcomeres prior to the
appearance of CapZ at nascent Z-discs. The distribution of CapZ and sa
rcomeric alpha-actinin in young myotubes was not coincident; in older
myotubes, both CapZ and alpha-actinin were co-localized at Z-discs. In
cardiac myocytes, CapZ was detected at Z-discs and was distributed in
a punctate pattern throughout the cytoplasm. CapZ also was co-localiz
ed with A-CAM and vinculin at cell-cell junctions formed by the myocyt
es. (C) 1993 Wiley-Liss, Inc.