Rab11a is modified in vivo by isoprenoid geranylgeranyl

Post-translational adduction of farnesyl or geranylgeranyl moieties to a terminal cysteine residue of proteins is a characteristic feature of ras-related GTP-binding proteins. According to current rules of prenylation, the carboxyl-terminal motif (CXXX or CC/CXC) as well the context of the cysteine residue dictate the extend and specificity of the isoprenoid modification. Rablla, a small GTP-binding protein that is associated with pathways regulating protein traffic, terminates with isoleucine at its C-terminus, suggesting that it may only be geranylgeranylated. Recent finding, however, showed that rab11a can be modified in vitro by different prenyl groups: farnesyl and geranylgeranyl [1]. To determine whether rablla is modified in vivo by both isoprenoids we transiently overexpressed rablla in COS1 cells, and analyzed the translation products by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) in combination with metabolic labeling in the presence of either [3H]farnesyl-PP or [3H]geranylgeranyl-PP. Contrary to the in vitro results, our studies showed that rab11a is post-translationally modified in vivo only by geranylgeranyl isoprenoid. The data implied that in vivo there must exist other determinant(s) that are necessary for prenyltransferase recognition.