SmgGDS: An Emerging Master Regulator of Prenylation and Trafficking by Small GTPases in the Ras and Rho Families

Abstract

Newly synthesized small GTPases in the Ras and Rho families are prenylated by cytosolic prenyltransferases and then escorted by chaperones to membranes, the nucleus, and other sites where the GTPases participate in a variety of signaling cascades. Understanding how prenylation and trafficking are regulated will help define new therapeutic strategies for cancer and other disorders involving abnormal signaling by these small GTPases. A growing body of evidence indicates that splice variants of SmgGDS (gene name RAP1GDS1) are major regulators of the prenylation, post-prenylation processing, and trafficking of Ras and Rho family members. SmgGDS-607 binds pre-prenylated small GTPases, while SmgGDS-558 binds prenylated small GTPases. This review discusses the history of SmgGDS research and explains our current understanding of how SmgGDS splice variants regulate the prenylation and trafficking of small GTPases. We discuss recent evidence that mutant forms of RabL3 and Rab22a control the release of small GTPases from SmgGDS, and review the inhibitory actions of DiRas1, which competitively blocks the binding of other small GTPases to SmgGDS. We conclude with a discussion of current strategies for therapeutic targeting of SmgGDS in cancer involving splice-switching oligonucleotides and peptide inhibitors.

Keywords: GDF; GEF; Rap1GDS1; SmgGDS; lipidation; prenylation; splicing; trafficking.

FIGURE 1

Schematic illustration depicting how the SmgGDS splice variants, SmgGDS-607 and SmgGDS-558, interact with pre-prenylated and prenylated small GTPases, respectively. (A) SmgGDS-607 has 13 ARM domains named A–M. SmgGDS-607 binds pre-prenylated small GTPases because the presence of ARM domain C inhibits the formation of a hydrophobic pocket in SmgGDS-607. In contrast, SmgGDS-558 lacks ARM domain C, causing it to have only 12 ARM domains. SmgGDS-558 binds prenylated small GTPases because a hydrophobic pocket forms between ARM domains B and D, which accommodates the prenyl group of small GTPases. (B) Homology models indicate that SmgGDS-558 binds different Ras and Rho family members in a similar manner, suggesting that these small GTPases compete for binding to SmgGDS-558. SmgGDS-558 is depicted with a gray surface plot. The small GTPases are depicted with an electrostatic surface plot with negative charges indicated by red and positive charges indicated by blue [homology models adopted from Bergom et al. (2016)].

FIGURE 2

Model depicting how SmgGDS splice variants regulate the prenylation and trafficking of small GTPases. (A) SmgGDS-607 binds a newly synthesized small GTPase and retains it until the correct signal causes SmgGDS-607 to release the pre-prenylated GTPase to the PTase. (B) SmgGDS-558 escorts newly prenylated small GTPases to the ER for post-prenylation processing. (C) SmgGDS-558 escorts prenylated and fully processed small GTPases from the ER to the plasma membrane. (D) Both SmgGDS splice variants might assist in nucleocytoplasmic shuttling of small GTPases (red arrows).

FIGURE 3

Schematic illustration depicting how unidentified GEFs and GDFs might release small GTPases from SmgGDS splice variants in the cytoplasm and at membranes (A–C), and in the nucleus (D–F). The interactions of these proteins with SmgGDS will control when the small GTPases will be prenylated or undergo post-prenylation processing, and determine where the small GTPases will localize in the cell.

FIGURE 4

Schematic illustration depicting the regulation of SmgGDS expression by the splice-switching oligonucleotide, SSO Ex5. (A) SmgGDS RNA contains 15 exons, and exon 5 encodes ARM domain C. Inclusion of exon 5 in mature SmgGDS mRNA generates SmgGDS-607, whereas omission of exon 5 in mature SmgGDS mRNA generates SmgGDS-558. (B) In cancer cells, the binding of unspecified spliceosome proteins to SmgGDS RNA promotes exon 5 inclusion and generates more SmgGDS-607 than SmgGDS-558. (C) Binding of SSO Ex5 to SmgGDS RNA promotes exon 5 skipping, generating more SmgGDS-558 than SmgGDS-607. Additional manuscript sections.