Calponin is a smooth muscle-specific, thin filament-associated protein
which has been implicated in the regulation of contraction via its in
teraction with actin and inhibition of the cross-bridge cycling rate.
Calponin is phosphorylated by protein kinase C (PKC) and Ca2+/calmodul
in-dependent protein kinase II (CaM kinase II), primarily at S175, wit
h loss of actin binding and inhibition of the actin-activated myosin M
gATPase. We previously isolated calponin phosphatase from chicken gizz
ard smooth muscle and identified it as a type 2A protein phosphatase [
Winder et al. (1992) Biochem. J. 286, 197-203]. The methods used to de
tect phosphatase activity in that study would additionally have detect
ed type 1 and 2C phosphatases, but not type 2B phosphatase (Ca2+/CaM-d
ependent phosphatase or calcineurin). We have, therefore, examined the
expression of type 2B phosphatase in smooth muscle and its ability to
dephosphorylate calponin. Western blotting with polyclonal antibodies
to the brain enzyme revealed the expression of type 2B phosphatase in
chicken gizzard, and immunofluorescence microscopy confirmed the pres
ence of the phosphatase in isolated smooth muscle cells (rabbit and to
ad stomach). The purified brain phosphatase dephosphorylated calponin
(phosphorylated by PKC or CaM kinase II) in a Ca2+/CaM-dependent manne
r. Dephosphorylation by calcineurin restored actin-binding and actin-a
ctivated myosin MgATPase inhibition which had been reduced by PKC-cata
lyzed phosphorylation. We conclude that calponin dephosphorylation may
be catalyzed not only by type 2A phosphatase but also by type 2B phos
phatase, raising the possibility that both phosphorylation and dephosp
horylation of calponin could be regulated by Ca2+/CaM.