N-Terminal Lipid Modification Is Required for the Stable Accumulation of CyanoQ in Synechocystis sp. PCC 6803

Abstract

The CyanoQ protein has been demonstrated to be a component of cyanobacterial Photosystem II (PS II), but there exist a number of outstanding questions concerning its physical association with the complex. CyanoQ is a lipoprotein; upon cleavage of its transit peptide by Signal Peptidase II, which targets delivery of the mature protein to the thylakoid lumenal space, the N-terminal cysteinyl residue is lipid-modified. This modification appears to tether this otherwise soluble component to the thylakoid membrane. To probe the functional significance of the lipid anchor, mutants of the CyanoQ protein have been generated in Synechocystis sp. PCC 6803 to eliminate the N-terminal cysteinyl residue, preventing lipid modification. Substitution of the N-terminal cysteinyl residue with serine (Q-C22S) resulted in a decrease in the amount of detectable CyanoQ protein to 17% that of the wild-type protein. Moreover, the physical properties of the accumulated Q-C22S protein were consistent with altered processing of the CyanoQ precursor. The Q-C22S protein was shifted to a higher apparent molecular mass and partitioned in the hydrophobic phase in TX-114 phase-partitioning experiments. These results suggest that the hydrophobic N-terminal 22 amino acids were not properly cleaved by a signal peptidase. Substitution of the entire CyanoQ transit peptide with the transit peptide of the soluble lumenal protein PsbO yielded the Q-SS mutant and resulted in no detectable accumulation of the modified CyanoQ protein. Finally, the CyanoQ protein was present at normal amounts in the PS II mutant strains ΔpsbB and ΔpsbO, indicating that an association with PS II was not a prerequisite for stable CyanoQ accumulation. Together these results indicate that CyanoQ accumulation in Synechocystis sp. PCC 6803 depends on the presence of the N-terminal lipid anchor, but not on the association of CyanoQ with the PS II complex.

The Photosystem II Assembly Factor Ycf48 from the Cyanobacterium Synechocystis sp. PCC 6803 Is Lipidated Using an Atypical Lipobox Sequence

“…The role of the lipid moiety in the function of Psb27, CyanoP, CyanoQ, and Ycf48 is not clear, although Juneau et al have concluded that lipidation may stabilize the CyanoQ protein [ 25 ]. Since PSII biogenesis seems to occur in specific regions of the thylakoid membranes, lipids may help maintain the location of assembly factors within these regions.…”

Mass spectrometry analysis of the photosystem II assembly factor Psb27 revealed variations in its lipid modification

“…Proteins might be tethered to the membrane just to limit diffusion in a two-dimensional space, which can be also mediated by a transmembrane helix (e.g. evolved from a former signal peptide) or the lipid moiety might play an additional role in the localization of the protein in a specific membrane or part of a membrane (Ferguson 1991;Okuda and Tokuda 2011;Juneau et al 2016). For photosynthesis associated CyanoQ, the lipidation is essential for processing and accumulation of the protein in the thylakoids, independent of the signal peptide itself, which emphasizes the importance of the process (Juneau et al 2016).…”