Geranylgeranyltransferase II
Geranylgeranyltransferase IIPrenylation of proteins is a prevalent post-translational modification in eukaryotic organisms. It confers a lipophilic nature to the modified protein via a covalent attachment of a polyisoprenoid group, typically a farnesyl (C15) or a geranylgeranyl (C20), to the sulfhydryl group of a cysteine residue near the carboxyterminus (1,2). In some proteins, modifications such as these are essential for accurate protein interactions as well as the correct membrane localization. Key signaling proteins such as p21ras, of which the oncogenic forms are involved in 20-30% of all human cancers, as well as various G-proteins, require prenylation for function (3,5).
G-proteins of the Rab family are of extreme importance in intracellular trafficking, and double geranylgeranylation of these proteins is essential for biological activity. Isoprenoid groups are transferred to cellular proteins by the small family of protein prenyltransferases. Farnesyltransferase (FTase) and geranylgeranytransferase I (GGTase I) are members of this group that recognize the CAAX prenylation signal. A third member, geranylgeranyltransferase II (GGTase II) or Rab geranylgeranyltransferase (RabGGTase) recognizes two C-terminal cysteines instead of the CAAX box, and requires the chaperon, Rab escort protein (REP-1) for recognition of substrate proteins and delivery of newly prenylated proteins to the target membrane after release from RabGGTase (1).
Focus on Rab prenylation increased with the
observation that Hermansky-Pudlak syndrome (HPS) and the X-linked
choroideremia (CHM) disease are a result of deficiences in Rab prenylation
due to various mutations in the genes encoding the enzyme complex (7).
The protein crystal structure of an isoprenoid bound RabGGTase-REP-1 complex
at 2.7Å indicated that the catalytic RabGGTase exists as a tightly
associated alpha/beta heterodimer of 68 kDa and 48 kDa respectively, and
the active site, located on the beta subunit has a zinc ion, and becomes
occupied by the isoprenoid moiety. The Rab binding platform is located
on the 75 kDa REP-1 protein which exhibits protein-protein interaction
with the alpha helix over an unusually small contact area. This is
observed in very few proteins and is remarkably similar to that observed
with the HIV gp120 envelope interaction with the CD4 receptor (6).
References
1. Casey PJ, Seabra MC.Protein
prenyltransferases.(1996) J Biol Chem. 271(10):5289-92.
2. Crowell DN.(2000) Functional
implications of protein isoprenylation in plants. Prog Lipid Res. 39(5):393-408.
3. Gelb MH, Scholten JD, Sebolt-Leopold
JS. (1998) Protein prenylation: from discovery to prospects for cancer
treatment. Curr Opin Chem Biol. 2(1):40-8.
4. Kalinin A, Thoma NH, Iakovenko
A, Heinemann I, Rostkova E, Constantinescu AT, Alexandrov K.(2001) Expression
of mammalian geranylgeranyltransferase type-II in Escherichia coli and
its application for in vitro prenylation of Rab proteins. Protein Expr
Purif. 22(1):84-91.
5. Nguyen DN, Stump CA, Walsh
ES, Fernandes C, Davide JP, Ellis-Hutchings M, Robinson RG, Williams TM,
Lobell RB, Huber HE, Buser CA.(2002) Potent inhibitors of farnesyltransferase
and geranylgeranyltransferase-I. Bioorg Med Chem Lett. 12(9):1269-73.
6. Pylypenko O, Rak A, Reents
R, Niculae A, Sidorovitch V, Cioaca MD, Bessolitsyna E, Thoma NH, Waldmann
H, Schlichting I, Goody RS, Alexandrov K.(2003) Structure of Rab
escort protein-1 in complex with Rab geranylgeranyltransferase. Mol Cell.
11(2):483-94.
7. Rak A, Reents R, Pylypenko
O, Niculae A, Sidorovitch V, Thoma NH, Waldmann H, Schlichting I, Goody
RS, Alexandrov K. Crystallization and preliminary X-ray diffraction analysis
of the Rab escort protein-1 in complex with Rab geranylgeranyltransferase.(2001)
J Struct Biol. 136(2):158-61.