Current studies derived from live cells injected with fluorescently labeled calmodulin reveal that this protein is evenly distributed throughout the pollen tube cytoplasm (Moutinho et al. 1998b). Whereas total calmodulin may not accumulate in the apex, that which is activated by binding to Ca2+ is elevated in a pattern that is similar to the tip-focused gradient (Rato et al. 2004). These findings have been gained through the use of TA-CaM, a fluorescent analog of calmodulin that changes its quantum yield when bound to Ca2+. These results are not surprising since presumably the tip-focused gradient is sufficient to saturate appropriate binding molecules such as calmodulin. However, what are the interacting proteins to which calmodulin binds? One example would be the actin binding proteins that are modulated by Ca2+ and calmodulin, including myosin and villin. Taken together it can be appreciated why there are not organized bundles of actin in the extreme apex and why streaming is markedly suppressed. A second example is ACA9 (Schi0tt et al. 2004), an au-toinhibited, plasma membrane-localized Ca2+ pump, which is thought to be regulated by calmodulin, an observation consistent with the well-known role of calmodulin in the regulation of ion pumps in other systems (Snedden and Fromm 2001) (Fig. 4). We suspect that there are many other response elements that respond to Ca2+/calmodulin, and that their identity will emerge from future work.
Recent work also supports the idea that calmodulin may interact with cyclic AMP in the regulation of pollen tube growth (Moutinho et al. 2001; Rato et al. 2004). Agents that either substitute for cAMP (8-Br-cAMP) or ac tivate adenylyl cyclase (forskolin) cause an increase in activated calmodulin, while inhibition of adenylyl cyclase (dideoxyadenosine) induces a decline in activated calmodulin. Rato et al. (2004) suggest that these interacting pathways participate in the regulation of apical secretion.
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