PARENT SESSION

Symposium S7C Biosynthesis and assembly: Protein trafficking
Thursday September 2nd, 2004 2:40 PM-4:40 PM Room 510B
Chair: Ken Cline
Co-Chair: Steven Theg

Posttranslational Protein Targeting to the ALB3 Translocase by a Signal Recognition Particle in Chloroplasts. Ralph Henry^*,1, ¹ Biological Sciences Dept., Fayetteville, AR, U.S.A.

ABSTRACT- Signal recognition particles (SRP) in the cytosols of prokaryotes and eukaryotes are used to target proteins to the cytoplasmic membrane and the endoplasmic reticulum, respectively. The mechanism of targeting relies on cotranslational SRP binding to hydrophobic signal sequences. An SRP in chloroplasts (cpSRP) is unusual in that it functions post-translationally to localize a subset of nuclear encoded thylakoid proteins, the LHCs. Stromal cpSRP, composed of a conserved 54 kDa GTPase (cpSRP54) and a unique 43 kDa polypeptide (cpSRP43), binds LHCs to form a soluble cpSRP-LHC targeting complex. Binding of cpSRP and the SRP receptor homologue cpFtsY to GTP and to each other at the thylakoid initiates the membrane phase of the targeting pathway that leads to their association with the integral membrane protein ALB3. ALB3, which is homologous to Oxa1p in mitochondria and YidC in bacteria, is required for LHC integration. GTP hydrolysis releases cpSRP54 and cpFtsY from each other and from ALB3 to initiate new rounds of targeting/insertion. The unique ability of cpSRP to bind full-length substrates stems from cpSRP43, which interacts with a conserved 18 amino acid motif in LHCs termed L18. However, little is known regarding the function of cpSRP43 beyond its role in promoting formation of a cpSRP-LHC targeting complex. Recent studies indicate that specific functional domains within cpSRP43 operate at distinct steps in the posttranslational cpSRP targeting pathway, including steps that regulate the GTPase activities of cpSRP54/cpFtsY. Based on studies of co-translational protein targeting by cytosolic SRPs, we hypothesize that release of LHCs from cpSRP to ALB3 at the thylakoid is tightly regulated through protein interactions that influence GTP binding and hydrolysis by cpSRP54 and cpFtsY. We further hypothesize that protein interactions mediated by cpSRP43 serve to regulate the timing of GTP binding and hydrolysis by cpSRP54 and cpFtsY.

KEY WORDS: Albino3, signal recongition particle, protein targeting, light harvesting complex