| Microalgal photosynthesis accounts for a significant portion of the carbon capture and sequestration on earth and influences biomass production and the global environment. Many microalgae species possess a CO2-concentrating mechanism (CCM) to cope with the limited CO2 supply in their natural habitats. The observation that a lesion in the LCIB gene in Chlamydomonas directly affects internal Ci accumulation demonstrates the important role played by its gene product, LCIB, in the Chlamydomonas CCM. Three additional homologues of LCIB are found in the Chlamydomonas genome, LCIC, LCID and LCIE. Multiple lines of evidence suggested that LCIB and LCIC form a complex: western blots reveal that LCIC, even though actively transcribed, is down-regulated at the protein level in the LCIB mutants, ad1 and pmp1; Screening of LCIB-interacting proteins using a yeast two-hybrid system revealed an apparently strong LCIB-LCIC interaction; and LCIC was co-purified with LCIB by in vivo affinity purification. A specific domain in LCIB appears to be important for LCIB-LCIC interaction and may distinguish LCIB functionally from the very similar LCIC. Immunofluorescence localization indicates a plastid localization for LCIB/LCIC, which is consistent with the prediction of putative N-terminal plastid signal peptides in these proteins. Re-localization of LCIB between a dispersed stromal distribution and a condensed distribution around the pyrenoid was observed in response to changes in CO2 concentration. We propose that LCIB is an integral component of a multi-subunit plastid complex, and that its function relies on its interaction with other proteins. Based on the currently published genome sequences, LCIB-like genes also exist in several other species of green microalgae, cyanobacteria and bacteria, suggesting an ancient origin of the LCIB gene family and its widespread occurrence in microorganisms. |