Open questions: what about the ‘other’ Rho GTPases?

Rho GTPases have many and diverse roles in cell physiology, and some family members are very well studied, including RhoA, Rac1 and Cdc42. But many are relatively neglected, and fundamental questions about their mechanisms and functions remain open.

know if they interact with and/or regulate the activity of other family members. Indeed, by studying only RhoA, Rac1 and Cdc42, we are likely to be missing the real functions of many GEFs and GAPs because their targets in cells are among the other Rho GTPases.
Interestingly, four family members-Rnd1, Rnd2, Rnd3 and RhoH-are 'atypical', in that they are known to be constitutively GTP-bound and do not hydrolyse GTP: much less is known about how these family members are regulated. RhoU and RhoV have high intrinsic GDP/ GTP exchange rates, so are unlikely to need GEFs for activation but could still be turned off by GAPs [1,4]. For two other members, RhoBTB1 and RhoBTB2, the Rho domains are quite divergent in sequence from other family members and they are unlikely to be regulated by GEFs or GAPs [5].
Information on other mechanisms to modulate Rho GTPases is missing for most of the family. For example, RhoA and Rac1 proteins are targeted for degradation following ubiquitination, but few other members of the family have been tested for ubiquitination [4]. In addition, RhoA and Rac1 have been highly studied for effects of phosphorylation on their activity, but apart from the regulation of Rnd3 by phosphorylation, little is known about how phosphorylation affects other Rho GTPases or even if they are phosphorylated [4].

How do they signal in cells?
When bound to GTP, Rho GTPases interact with their downstream targets to induce cellular responses (Fig. 1). As described above, post-translational modifications can modulate their activity. Many downstream targets for RhoA, Rac1 and Cdc42 have been identified by a combination of biochemical, molecular and bioinformatics approaches, and structural studies have clearly elucidated the mechanisms of interaction for some [6]. The interactions of other Rho GTPases with known RhoA, Rac1 or Cdc42 targets have been tested individually, but in most cases no concerted approach has been taken to identify their targets in the way that targets have been identified for RhoA, Rac1 and Cdc42. The exception is the Rnd proteins, for which several new targets have been identified by yeast two-hybrid screening, although so far we know relatively little about the functional consequences of these interactions [7]. RhoBTB proteins have been shown to interact via their BTB domains with Cullin-3, a scaffold for ubiquitin ligase complexes, but again the functional role of this interaction is not clear [5]. A challenge for the future will be to identify the protein interactome for each Rho GTPase and to determine how their interactions change dynamically during cellular responses and vary between cell types and with external conditions.

What are their cellular and organismal functions?
While the effects of RhoA, Rac1 and Cdc42 on cytoskeletal dynamics, cell migration, intracellular membrane transport and cell polarity are well established [8], there are other Rho GTPases for which we have relatively little information about their physiological roles [1]. Some have been more comprehensively studied in specific cell types. For example, Rnd3 has been shown to reduce actomyosin contractility via its interaction with a GAP for RhoA, p190RhoGAP, and, hence, to contribute to migration of mouse neurons in vivo [9]. RhoH is known to contribute to T-cell receptor signalling and, hence, affects T-cell development and function in mice [10]. Studies on the roles of other Rho family members in model organisms will provide new insight into their functions.