A series of thylakoid membrane proteins, including PsbX, PsbY and PsbW, are synthesized with cleavable signal peptides yet inserted using none of the known Sec/SRP/Tat/ Oxa1-type insertion machineries. Here, we show that, although superficially similar to Sec-type signal peptides, these thylakoidal signal peptides contain very different determinants. First, we show that basic residues in the N-terminal domain are not important, ruling out electrostatic interactions as an essential element of the insertion mechanism, and implying a fundamentally different targeting mechanism when compared with the structurally similar M13 procoat. .
