Abstract
The cilia of P a r a m e c i u m carry an excitable membrane allowing the influx of Ca 2+ through voltage-sensitive Ca 2+ channels [1]. The increased internal Ca 2+ concentration caused by this Ca 2+ transient produces a change in the ciliary power stroke and hence in the direction of swimming [1 ]. Using P a r a m e c i u m as a model to study the biochemistry of the excitable ciliary membrane we have demonstrated the presence of a guanylate cyclase (GC, EC 4.6.1.2) in this membrane [2]. It has also been shown that the GC is a likely intraciliary target of the incoming Ca 2÷-signal [3]. The activity of this enzyme is regulated by concentrations of Ca 2+ ions well within the physiological range, half maximal activity is seen with 8 t~M Ca 2+ [3]. Ca 2+ ions have been proposed as a second messenger and implicated in a great variety of cellular functions. Recent evidence indicates that a family of homologous proteins (e.g. calmodulin (CAM), troponin C, myosin light chains and parvalbumins) are the most common mediators of the intracellular effects of Ca 2+ . Particularly CaM has a wide distribution within eukaryotic cells and activates a number of different enzymes [4]. Here we present evidence that CaM is a component of the GC of ciliary membranes from Paramec ium. A particularly interesting feature of this enzyme system is the finding that very low concentrations of La 2+ dissociate the tight coupling between CaM and GC.
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call