Function

Vav1 is a Guanine Exchange Factor (GEF) for the Rho/Rac protein family ^[1]. It is involved in cellular response to external stimuli, by inducing cytoskeletal rearrangement or by inter-mediating pathways cascades. As such, the proto-oncogene encoded Vav1 protein has a role in growth and division of the cell, but also in apoptotic pathways.

Vav1 is mainly found in hematopoietic cells ^[2] and is involved in the differentiation of T lymphocytes, then in B and T cells development and maturation ^[3]. Vav1 also has a role in Ca2+ induced signalling pathways in immune response of B and T cells.

HIV-1 Nef protein has been found to bind specifically to Vav1, affecting its nucleus-to-cytoplasm distribution, and thus delaying the immune reaction ^[4]

Disease relevance

VAV1 is often considered a potentially prooncogenic protein since anormal VAV1 was proved to be involved in many cases of mammalian malignancies, including neuroblastoma, melanoma, pancreatic, lung and breast cancers^[5]. What's more, it's also often found to be overexpressed in cancer cells whereas it's usually only expressed in the hematopoietic system. VAV1 is indeed involved in many pathways that ultimalty lead to the regulation of the activity of several transcription factors that play a major role in the regulation of cell apoptosis or cell proliferation, such as NF-AT^[6] or Rac1, PAK1, and NF-κB that causes cyclin D1 upregulation^[7]. It's still unknown if mutations take part in the cell proliferation effect. What has been confirmed though, is that mere epigenetic modifications could contribute to it. Furthermore, VAV1 is implicated in other immune system related pathways. Studies showed that VAV1 deficient organisms had a slower immune response to infections. No diseases have been found out though that was involving such a feature of VAV1^[8].

Therapeutic context

Although VAV1 has clearly been identified as an oncogenic protein already, there are still very little ways to regulate its activity as a cell proliferation factor. Treatments are being studies for the moment. Studies showed that VAV1 promoter demethylation as an event in cancer progression, suggesting that aberrant signaling pathways driven by VAV1 are potential therapeutic targets in Pancreatic ductal adenocarcinoma^[9]. Aside from that, it's also being researched if already known antioncogenic drug would stop VAV1 induced cell proliferation^[10].

Structural highlights

The Vav1 are (from N ter to C ter): a calponin-homology (CH) domain, an acidic (Ac) domain, a DBL-homology (DH) domain also known as a rhoGEF domain, a pleckstrin-homology (PH) domain, a phorbol esters/diacylglycerol binding (C1) domain also called Zinc-finger domain and Src-homology (SH) 3 and 2 domains. ^{[11] [12]}

(residues 1 - 119): When the CH domain is deleted, Vav has a partial phosphorylation-independant GEF activity. ^[13]

(residues 134 - 187): contains the Tyrosine (Y174) which activates Vav when phosphorylated. ^[14]

(residues 194 - 373): present in every Rho/rac GEF, it is the active site of the GTP exchange with the C1 domain in Vav and it promotes the binding to rho GTPase with the help of the C1 domain. It always has a PH domain just after its C terminal.^[15]

(residues 402 - 504): point mutation in it don't change the exchange activity of Vav, so it isn't directly involved in its GEF function ^[16]. But the presence of phosphatidylinositol-4,5-bisphosphate (PIP 2) inhibits 90% of Vav activity while phosphatidylinositol-3,4,5-trisphosphate doubles it. It is very likely that the interactions between these lipids and Vav happen on the PH domain, as it is commonly the case on other proteins with this region. It should be noted that the activation of Vav is made in parallel of the one of PI3-kinase in TCR and BCR pathway. ^[17]

(508 - 584) : has a cooperative action with the DH domain to bind to the rho GTPase and to promote the GEF activity of Vav.

Src-homology domains SH3-SH2-SH3 (residues 617 - 842): the activators of Vav like Syk or ZP-70 fix on its SH2 region ^[18]. Potential inhibitors of Vav also dock on its SH3-SH2-SH3 region, such as SHP which would remove the phosphate on Y174 ^[19] or Cbl-b ^[20]. It has also been proven that the C-ter SH3 was involved in the auto-inhibition of Vav. ^[21]

Auto-inhibition

When Tyr-174 on the Ac domain is not phosphorylated, Vav1 is auto-inhibited. It has long been thought that only the CH and Ac domain interactions with the DH, PH and C1/ZN domains played a role in auto-inhibition. Indeed, , forming beta-sheet like hydrogen bonds. There is also a interaction between the PH and Ac domains, some oppositely charged residues facing each other like Asp-150 on Ac and Lys-487 on PH. These interactions seem to strengthen the formation of an inhibitory helix on the Ac domain (residues 167-178) which binds on DH domain and block its access to the substrate. Moreover, the interactions between the PH and CH domains coupled with the interaction of the latter and the N-terminus region of the Ac domain strengthen the helix, so there are two processes involved in Vav auto-inhibition. ^[22]

Tyr-541 and Tyr 544 on C1 and Tyr-836 on the C-terminal SH3 domains have also to get phosphorylated in order to activate Vav1, the SH3 domain also binding on the DH (residues 371 – 388) if this tyrosine wasn’t phosphorylated. ^[23]

Activation

The phosphorylation of Tyr-174 in vivo happens within seconds after a kinase has bound to Vav1. It induces the deformation of the inhibitory helix, thus et separates itself of the active site of the DH domain. The phosphorylation of Tyr-160 and -142 makes Tyr-174 more accessible as it lessen the interactions between the CH and Acidic domains. ^[24]

Vav activity

Upon antigen recognition in BCR and TCR pathways, Src family kinases phosphorylate Y174 residue of Vav1, so that activated Vav1 can then express its GEF activity on Rac1 ^[25]. The PH domain structures the protein so that DH and C1 are available for Rac1 binding. The C1 domain is rich in cysteine and harbours two Zn fingers involved in the binding of RhoGTPases ^[26].

Rac1 has two switch regions that bind to C1 and DH domain. The first switch binds to two highly conserved regions of C1 domain, CR1 (Conserved Region)and CR3. The second switch binds to CR3 and to the C-terminal α-helix of DH domain. These two domains widen the binding cavity to mediate the release of GDP to exchange for GTP ^[27].

Vav family is atypical for its special C1 domain with Zn fingers. This allows recognition of various RhoGTPases, while GEFs proteins are usually specific to only one.