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knockout (KO) mouse, however, has shown that α-smooth actin is dra-
matically upregulated in the brain and localized to cells morphologically
resembling neurons (
Cheever et al., 2012
). Nevertheless, given that multiple
studies have reported a predominance of β- and γ-cytoplasmic actin in
roughly equal amounts within brain and isolated neuronal populations, we
will focus on these isoforms, henceforth simply referred to as β- and γ-actin.
3.2. Distinct Biochemical Properties
Despite their nearly identical amino acid sequences, multiple groups have
independently generated antibodies that are able to distinguish between
β- and γ-actin, and revealed striking yet sometimes conflicting reports of
distinct subcellular localization patterns. Early work in fibroblasts demon-
strated that β-actin was often enriched at the leading edge of migrating
cells, whereas γ-actin appeared to be concentrated within stress fibers and
the perinuclear region (
Hoock et al., 1991
;
Kislauskis et al., 1997
;
Otey
et al., 1986
;
Shestakova et al., 2001
). In addition, overexpression of β-actin
in myoblasts led to an increase in cell migration and surface area, whereas
overexpression of γ-actin promoted cell retraction and rounding (
Schevzov
et al., 1992
). Collectively, these early studies suggested a specific role for
β-actin in promoting cell motility at the leading edge, whereas γ-actin
appeared to have a more structural or contractile role. While this apparent
preferential subcellular sorting of actin isoforms is a widely held view, it is
important to note that not all studies have agreed on this critical issue. Addi-
tional work with newly generated monoclonal antibodies reported essen-
tially the opposite localization pattern, with β-actin preferentially enriched
in stress fibers while γ-actin appeared to be more ubiquitously localized in
multiple different cell lines (
Dugina et al., 2009
). Using an independent set
of actin-isoform-specific antibodies verified on β- and γ-actin KO tissues
and cells, our group has observed complete colocalization of β- and γ-actin
in every tissue and cell type examined to date (
Bunnell et al., 2011
;
Cheever
et al., 2012
;
Perrin et al., 2010
). Possible explanations for the discrepancies
in localization patterns are discussed in detail in Section
4.1
.
Recent biochemical studies from the Rubenstein group have provided
additional evidence for the widely held hypothesis that β-actin is the more
dynamic actin isoform and preferentially contributes to cell motility and
dynamics. Using a baculovirus expression system,
Bergeron et al. (2010)
were able to characterize the unique biochemical properties of β- and
γ-actin. In the Mg
2+
bound form of actin generally regarded to be the most
physiologically relevant, both cytoplasmic actin isoforms polymerize much
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