Distinct albino3-dependent and -independent pathways for thylakoid membrane protein insertion

The homologous proteins Oxa1, YidC, and Alb3 mediate the insertion of membr ane proteins in mitochondria, bacteria, and chloroplast thylakoids, respect ively. Depletion of YidC in Escherichia coli affects the integration of eve ry membrane protein studied, and Alb3 has been shown previously to be requi red for the insertion of a signal recognition particle (SRP)-dependent prot ein, Lhcb1, in thylakoids. In this study we have analyzed the "global" role of Alb3 in the insertion of thylakoid membrane proteins. We show that inse rtion of two chlorophyll-binding proteins, Lhcb4.1 and Lhcb5, is almost tot ally blocked by preincubation of thylakoids with anti-Alb3 antibodies, indi cating a requirement for Alb3 in the insertion pathway. Insertion of the re lated PsbS protein, on the other hand, is unaffected by Alb3 antibodies, an d insertion of a group of SRP-independent, signal peptide-bearing proteins, PsbX, PsbW, and PsbY, is likewise completely unaffected. Proteinase K is f urthermore able to completely degrade Alb3, but this treatment does not aff ect the insertion of these proteins. Among the thylakoid proteins studied h ere, Alb3 requirement correlates strictly with a requirement for stromal fa ctors and nucleoside triphosphates. However, the majority of proteins teste d do not require Alb3 or any other known form of translocation apparatus.