Abstract
Ca2+-dependent protein kinase (CDPK) has been proposed to mediate inhibition by Ca2+ of cytoplasmic streaming in the green algaChara. We have identified the in vivo substrate(s) of CDPK inChara by using vacuolar perfusion of individual internodal cells with [γ-32P]ATP. Phosphorylation of several polypeptides is enhanced when perfusions are performed at 10−4M free Ca2+ compared to <10−9M free Ca2+. The Ca2+-stimulated phosphorylation of these proteins is inhibited by the presence of a monoclonal antibody to soybean CDPK. One of these proteins is 16 to 18kDa and is recognized by an antibody against gizzard myosin light chains. These results demonstrate that inChara, several polypeptides are phophorylated by CDPK and one of these proteins has been tentatively identified as a myosin light chain. These observations support the hypothesis that Ca2+-regulated phosphorylation of myosin is involved in the regulation of cytoplasmic streaming.
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