How to design a highly organized cell: an unexpectedly high number of widely diversified SNARE proteins positioned at strategic sites in the ciliate, Paramecium tetraurelia.

There are only scattered data available on molecular aspects of vesicle trafficking in protozoa, notably in ciliates. In this context, proteins of paramount interest are the so-called SNARE proteins (soluble NSF attachment protein receptor; NSF=N-ethylmaleimide ~ensitive factor). They are positioned on opposite membranes; together with some other proteins they serve docking, e.g., of a yesic le to a target membrane (v-lt-SNAREs), and finally membrane fusion (Jackson and Chapman 2006; Jahn and Scheller 2006; Jahn et al. 2003; Malsam et al. 2008; Parlati et al. 2002; S611ner et al. 1993). In the cell, SNAREs contribute to the specificity of such interactions (Bethani et al. 2007; Parlati et al. 2002; Paumet et al. 2004), together with monomeric GTP-binding proteins (G-proteins, small GTPases [Grosshans et al. 2006; Rothman 1994]), the vesicular H+ -ATPase (V-ATPase [Hurtado-Lorenzo et al. 2006; Pfeffer 2007]), F-actin (Soldati and Schliwa 2006) and a variety of additional proteins (Wojcik and Brose 2007). The assembly and function of SNAREs during vesicle docking and membrane fusion is outlined in Figure 1. We now have systematically identified SNAREs in Paramecium tetraurelia and analyzed their distribution and basic aspects of their function (Kissmehl et al. 2007; Schilde et al. 2006, 2008, 2010), together with the SNARE-specific chaperone, NSF (Froissard et al. 2002; Kissmehl et al. 2002), in these cells. This is summarized here with the aim to set a baseline for future research on this fundamental aspect of vesicle trafficking. No comparably comprehensive analyses are available from any other free-living protozoan, but as far as possible such data are included, as are some data from algae. It is challenging to see how these data