Loss of Albino3 Leads to the Specific Depletion of the Light-Harvesting System

The chloroplast Albino3 (Alb3) protein is a chloroplast homolog of the mitochondrial Oxa1p and YidC proteins of Escherichia coli , which are essential components for integrating membrane proteins. In vitro studies in vascular plants have revealed that Alb3 is required for the integration of the light-harvesting complex protein into the thylakoid membrane. Here, we show that the gene affected in the ac29 mutant of Chlamydomonas reinhardtii is Alb3.1 . The availability of the ac29 mutant has allowed us to examine the function of Alb3.1 in vivo. The loss of Alb3.1 has two major effects. First, the amount of light-harvesting complex from photosystem II (LHCII) and photosystem I (LHCI) is reduced >10-fold, and total chlorophyll represents only 30% of wild-type levels. Second, the amount of photosystem II is diminished 2-fold in light-grown cells and nearly 10-fold in dark-grown cells. The accumulation of photosystem I, the cytochrome b 6 f complex, and ATP synthase is not affected in the ac29 mutant. Mild solubilization of thylakoid membranes reveals that Alb3 forms two distinct complexes, a lower molecular mass complex of a size similar to LHC and a high molecular mass complex. A homolog of Alb3.1 , Alb3.2 , is present in Chlamydomonas, with 37% sequence identity and 57% sequence similarity. Based on the phenotype of ac29 , these two genes appear to have mostly nonredundant functions.