DEPHOSPHORYLATION OF CALPONIN BY TYPE 2B PROTEIN PHOSPHATASE

Citation
Ed. Fraser et Mp. Walsh, DEPHOSPHORYLATION OF CALPONIN BY TYPE 2B PROTEIN PHOSPHATASE, Biochemistry, 34(28), 1995, pp. 9151-9158
Citations number
66
Categorie Soggetti
Biology
Journal title
ISSN journal
00062960
Volume
34
Issue
28
Year of publication
1995
Pages
9151 - 9158
Database
ISI
SICI code
0006-2960(1995)34:28<9151:DOCBT2>2.0.ZU;2-8
Abstract
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.