Golgi and Endoplasmic Reticulum

Ras proteins localise to both the Golgi and endoplasmic reticulum (ER). Differences are observed among the Ras isoforms. While both N-Ras and H-Ras localise to the plasma membrane, Golgi and the ER, K-Ras is more restricted and is found predominantly at the plasma membrane (Choy et al. 1999). With the development of markers, consisting of the Ras-binding domain of C-Raf linked to a fluorescent tag, which permit visualisation of GTP-bound Ras, active H-Ras was observed to localise at the Golgi and ER following growth-factor stimulation (Chiu et al. 2002). Interestingly, the kinetics of H-Ras activation differs among cellular compartments. At the plasma membrane and the ER, H-Ras activation is rapid, occurring within 1 min, and reversed after 20-40 min, while activation at the Golgi is delayed, taking

10 min, and persisting for 60 min (Chiu et al. 2002). Localisation to neither the Golgi nor ER is affected by inhibiting vesicular transport, suggesting that Ras is activated in situ by a diffusible factor. This premise has turned out to be correct, and the factor was identified as calcium (Bivona et al. 2003; Arozarena et al. 2004). Calcium activates distinct Ras GEFs on the Golgi (Bivona et al. 2003) and ER (Arozarena et al. 2004), resulting in activation of Ras, and thereby bypassing the typical Grb2/SOS pathway. Interestingly, calcium also activates the Ras GAP, CAPRI, at the plasma membrane (Lockyer et al. 2001), enabling differential activation/deactivation of Ras to occur in distinct cellular compartments.

A physiological role for MAPK signalling from the ER or the Golgi has been difficult to elucidate. Nevertheless, some difference in signalling between these two compartments has been demonstrated. Ras61L tethered to the Golgi is a strong activator of ERK and the Akt protein kinase (also known as protein kinase B), but weakly activates JNK. Conversely, when tethered to the ER, Ras61L strongly activates JNK, while only weakly activating ERK and Akt (Chiu et al. 2002). In Jurkat T cells, low-grade stimulation of the TCR upregulates the Ras GEF, Ras GRP1, and results in restriction of active N-Ras to the Golgi (Perez de Castro et al. 2004). The highly restricted activation suggests N-Ras signalling from the Golgi plays an important role in T-cell activation. Finally, an ER-confined Ras effector, ER-associated Ras inhibitor protein 1 (ERI1), was discovered in Saccharomyces cerevisae (Sobering et al. 2003). ERI1 binds to and inhibits GTP-Ras2p at the ER.

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