Figure 1.

Possible function of ICR1 and ROPs in localized exocytosis. (1) Activated, GTP-bound, ROPs are transiently acylated on an internal cysteine residue and as a result partition into detergent resistant membranes (DRM) that could be lipid rafts.¹³ (2) ICR1 monomer or dimer interacts with SEC3 in the cytoplasm and in turn this complex interacts with ROPs at the plasma membrane.²³ The ROP-ICR1-SEC3 complex could then serve a landmark for the formation of the exocyst complex. Alternatively, ICR1 may interact with SEC3-exocyst complex or may be a part of the exocyst complex or sub-complex. (3) Upon GTP hydrolysis the ICR1 is released from the ROP and the ICR1-SEC3-exocyst complexes return back to the cytoplasm. The ROP is deacylated and partitions into detergent soluble membranes. The rate of ROP activation/inactivation cycle depends primarily on ROP-GAPs²⁹ and ROP-GEFs and the possible activation/inactivation of the latter by receptor protein kinases in the plasma membrane.³⁰,³¹ In addition to prenylation and transient acylation, membrane localization of ROPs very likely depends on function of RhoGDIs³² and interaction of the C-terminal polybasic domains of ROPs with phosphatidylphosphoinositides (PIPs).³³