Nucleo cytoplasmic trafficking
c. Directionality of Transport: The RanGTPase System
In order to achieve effcient transport, the cargo has to be bound by the receptor in one compartment and released in the other compartment. This is accomplished by the loading state of small GTPase Ran. Ran exists in two flavours, the GTP bound form and the GDP bound form. Ran exhibits a low intrinsic hydrolysis activity and requires additional factors which aid in accelerating the hydrolysis. These proteins, so-called RanGAPs (GTPase Activating Proteins) reside in the cytoplasmic compartment either in a free state or bound to cytoplasmic extensions of the NPC. Moreover, the cell contains Ran Binding proteins (RanBPs, like RanBP1 and 2) further increasing the hydrolysis rate (Fig. 1). Once Ran has hydrolysed the GTP to GDP and inorganic phosphate, Ran has to be reverted to the GTP bound form by Ran gunaine nucleotide exchange factor RCC1 (regulator of chromosome condensation 1) into the GTP bound form. RCC1 is localized exclusively in the nuclear compartment. Thus Ran in the GTP bound form is predominantly in the nucleus, whereas the GDP bound form is mainly restricted to the cytoplasmic compartment. This steep gradient across the nuclear membrane is a prerequisite for efficient nuclear trafficking. RanGTP binds to receptors in the nucleus and traverses to the cytoplasm, where the hydrolysis of GTP by RanGAP triggers its release from the receptor. For a recharging with GTP it is actively transported back into the nucleus by NTF2 (nuclear transport factor 2)(Fig. 2).
Fig.1. Directionality of transport is mediated by the small GTPase Ran.
The Ran is loaded with GTP by the guanine nucleotide exchange factor RCC1 and its intrinsically low hydrolysis rate is triggered by the GTPase activating protein RanGAP and even more accelerated by binding of RanBPs (Binding Proteins).
The established gradient and the binding properties of Ran are also the cause for the directed transport of cargoes by the different receptors. Whereas importins bind their cargo only in the absence of Ran, exportins require the presence of Ran for cargo binding. As mentioned above all receptors bind to Ran in the GTP bound form. Thus importins bind their cargo in the cytoplasm traverse through the NPC and RanGTP triggers their dissociation and the RanGTP- bound importins are destined for export back into the cytoplasm (Fig. 3). On the contrary, exportins have the opposite properties. They bind the cargo only when RanGTP is present, ergo in the nucleus, and release it in the cytoplasm returning in the nucleus in an empty state, where upon binding to RanGTP an new cycle begins.
Fig.2. DA steep gradient of RanGTP across the nuclear envelope drives the directionality of transport.
RanGTP is required for the transfer of receptors belonging to the Importin-ß superfamily into the cytoplasm. After hydrolysis and dissociation from the receptor, RanGDP itself is shuttled back into the nucleus by NTF2.
Fig.3. Transport cycle for Importins and Exportins.
Importins bind their cargo in the cytoplasm and deliver it to the nucleus. Upon binding of RanGTP, the cargo is released and the importin is shuttled back into the cytoplasm. In contrast, exportins only bind their cargo in the presence of RanGTP, thus in the nucleus and release it in the cytoplasmic compartment after hydrolysis of RanGTP.
J Cell Sci. 2009 Mar 1;122(Pt 5):587-93. doi: 10.1242/jcs.015289. [Abstract]