The Regulatory RAB and ARF GTPases for Vesicular Trafficking

While highly conserved in structure and in funda- mental regulatory aspects for their activities, the RAS superfamily of monomeric GTP-binding proteins, or small GTPases, comprise a large family of regulatory molecules that collectively regulate diverse and critical cellular processes in eukaryotes. The RAB and ARF GTPases are members of two of the RAS-related sub- families that function in regulating vesicle trafficking, starting from regulating the formation of vesicles on donor membranes and directing trafficking specificity to and facilitating vesicle docking on target membranes (Zerial and McBride, 2001; Gillingham and Munro, 2007). Studies in yeast (Saccharomyces cerevisiae) and mammalian systems have shown that successive steps in endomembrane trafficking, from the endoplasmic reticulum (ER) to the Golgi, intra-Golgi, post-Golgi, and endocytic trafficking, are mediated by subfamilies of RAB and ARF GTPases. The fundamental roles that these small GTPases play in membrane trafficking are indicated by their conservation throughout eukaryotes and their proliferation in the more complex animal and plant systems. Structural conservation among these GTPases has facilitated a rather comprehensive iden- tification of the corresponding gene families in plants with complete genome information, in particular those for the Arabidopsis (Arabidopsis thaliana) RABs and ARFs (Pereira-Leal and Seabra, 2001; Rutherford and Moore, 2002; Vernoud et al., 2003; Zhang et al., 2007). In most cases examined, sequence conservation among yeast, mammalian, and plant RAB GTPases has corre- lated with similar localization on discrete subcellular compartments of the endomembrane trafficking sys- tems in these diverse organisms. However, while the most closely related RAB GTPases from diverse organ- isms appear to hold analogous positions within the endomembrane system, it is clear that experimental strategies directly targeted at examining the specific functions of these small GTPases in plants will be required to determine distinctions in how various membrane compartments are utilized and how plant- specific cargos are sorted and trafficked through these compartments. We refer readers to earlier reviews (Rutherford and Moore, 2002; Vernoud et al., 2003) and focus our discussion here on recent studies that examine the biological role for these small GTPases in specific cellular context as well as emerging studies that explore their broader regulatory networks.