Investigating RAB2 as a Vesicle Transporter & Autophagy Initiator

RAB2 is part of the RAB family of small GTPases that regulate intracellular trafficking of membrane bound vesicles. It is highly conserved in eukaryotes, from C. elegans to humans. These GTPases influence the creation, movement, attachment, fusion, and destiny of vesicles originating in the endoplasmic reticulum (ER)/Golgi systems. Like other members of its family, RAB2 cycles between an active (GTP carrying) and inactive (GDP carrying) state. RAB2 engages in transport between the ER and Golgi, associating with transport vesicles in accord with its C-terminal lipid modification. After docking the vesicle at the target membrane, RAB2’s GTPase hydrolyzes GTP to transition to the inactive form. As we learn more about intracellular vesicular traffic, RAB2 gains prominent roles in autophagy, lysozyme function, cancer, neurological disease, and diabetes.¹

RAB2 is a 212 amino acid, 23 kDa protein. Common to its family members, it shares two switch regions (I and II) in a conserved fold. The distinctive pattern of beta sheets and alpha helices in this fold enables guanine nucleotide and magnesium binding, in addition to the GTPase function. The switch regions dictate the interactions between RAB2 and various GTP/GDP exchange factors and GTPase-activating proteins. RAB2 ferries vesicles between the cytoplasmic faces of intracellular membranes of the ER and cis-Golgi. Its reversible membrane localization is specified by the post-translational prenylation of its very C-terminal cysteine residues by geranylgeranyl transferase. An escort protein conveys newly synthesized RAB2 to the transferase. This escort protein also chaperones the modified protein to the appropriate membrane. Specific factors escort both active GTP-loaded RAB2 to a given target membrane and inactive GDP-loaded RAB2 from its destination membrane.²

RAB2 in Autophagy

While RAB2 was originally found to transport proteins from the ER to the Golgi³, it has more recently been studied for its role in autophagy. It is, largely, a resident of the Golgi apparatus that is mobilized when the cell is under stress. Under these conditions, the Golgi sets RAB2 in motion to initiate phagophore formation by recruiting and activating ULK1. RAB2, then, interacts with RUBCNL and STX17 to become a fully active autophagosomal GTPase. At this point, it participates in recruiting the HOPS complex to facilitate autophagosome/lysosome fusion. From this role, RAB2 is mostly thought of as an aid in cellular recycling and waste disposal. However, recent work points to more ways that it influences several important (and age-related) disease states.⁴

The Expanding Impact on Many Disease States

Elevated levels of RAB2 expression have been found in both breast and cervical cancer.^5,6 In both types of cancer, increased expression was seen to enable metastasis, invasion, and accelerate epithelial-to-mesenchymal transition. DNA copy number amplifications and mRNA upregulation were the causes of increased expression. But why should this affect disease severity and prognosis? Elevation of RAB2 interferes with the processing, transport, and delivery of E-cadherin, a protein crucial to cellular cohesion and tissue integrity. Likewise, increased levels of RAB2 increase the export and excretion of membrane type 1 matrix metalloproteinase (MT1-MMP), dissolving extracellular proteins that maintain cellular attachments. In essence, oncogenic overexpression of RAB2A impairs the canonical Golgi-to-Plasma membrane transport of E-cadherin and accelerates the novel post-endocytic transport of MT1-MMP. Contrary to established paradigm, it seems epithelial cancers can and do select for factors that subvert normal vesicle trafficking to increase their own aggressiveness.⁷

In neurological diseases, it’s precisely the dearth of RAB2 that impairs synaptogenesis and neurotransmission. Loss of function mutations restrict synaptic vesicle proteins to the neural cell body and the trans-Golgi, compromising or destroying neural function. Ongoing studies are beginning to implicate this process in Autism Spectrum Disorders, Parkinson’s Disease, and Alzheimer’s Disease.⁸

In hyperglycemia, RAB2 has now been found to act in concert with poly (ADP-ribosyl)ated GAPDH to control insulin production, thereby, protecting pancreatic beta cells from excess ROS.⁹

As RAB2 is found to influence more vesicle and protein trafficking functions, its impact on disease states can only expand.

Study RAB2 with Transcreener’s GTPase Assay

View Transcreener GTPase Assay

References

1. Stenmark, H. and Olkkonnen, V.M. (2001) The Rab GTPase Family. Genome Biology, 2, reviews, 3007.1. Review. https://doi.org/10.1186/gb-2001-2-5-reviews3007.
2. Takai, Y. et al. (2001) Small GTP-Binding Proteins. Physiological Reviews, 81(10), 153-208. Review. https://doi.org/10.1152/physrev.2001.81.1.153.
3. Tisdale, E. J. et al. (1992) GTP-binding mutants of rab1 and rab2 are potent inhibitors of vesicular transport from the endoplasmic reticulum to the Golgi complex. Journal of Cell Biology, 119(4), 749-761. https://doi.org/10.1083/jcb.119.4.749.
4. Ding, X. et al. (2019) RAB2 regulates the formation of autophagosome and autolysosome in mammalian cells. Autophagy, 15(10), 1774-1786. https://doi.org/10.1080/15548627.2019.1596478.
5. Lin, S. et al. (2020) Comprehensive analysis of the value of RAB family genes in prognosis of breast invasive carcinoma. Bioscience Reports, 40(5), BSR20201103. https://doi.org/10.1042/BSR20201103.
6. Meng, Y. et al. (2022) RAB2A promotes cervical cancer progression as revealed by comprehensive analysis of HPV integration and proteome in longitudinal cervical samples. Clinical and Translational Medicine, 12(3), e767. Letter to the Editor. https://doi.org/10.1002/ctm2.767.
7. Kajiho, H. et al. (2018) Harnessing membrane trafficking to promote cancer spreading and invasion: The case of RAB2A. Small GTPases, 9(4), 304-309. https://doi.org/10.1080/21541248.2016.1223990.
8. Goetz, T.W.B. et al. (2021) Rab2 regulates presynaptic precursor vesicle biogenesis at the trans-Golgi. Journal of Cell Biology, 220 (5): e202006040. https://doi.org/10.1083/jcb.202006040.
9. Sugawara, T. et al. (2014) Rab2A is a pivotal switch protein that promotes either secretion or ER-associated degradation of (pro)insulin in insulin-secreting cells. Nature Scientific Reports, 4: 6952. https://doi.org/10.1038/srep06952.