Figure 4.

Crosslinking of pS-protA, pS-protA-1, or pS-protA-2 to envelope polypeptides at three stages in import. Energy- depleted chloroplasts (2 mg chlorophyll) were incubated with 200 nM urea-denatured pS-protA (pS-protA), pS-protA-1 (pS-protA-1), or pS-protA-2 (pS-protA-2) in the presence of apyrase (+ Apyrase), or the indicated concentrations of ATP (ATP) and GTP (GTP) at 26°C in the dark. The chloroplasts were irradiated at 312 nM for 20 min on ice with a transilluminator at a distance of 5 cm. Reisolated chloroplasts were fractionated to yield a total envelope membrane fraction. Envelope membranes (50 μg of protein) were analyzed by SDS-PAGE under reducing conditions and fluorography. The molecular masses of standard proteins are indicated at the center of the figures. The positions of pS-protA, S-protA, Toc86, Toc75, Toc34, Tic22, and Tic(21) are shown to the left and right of the figures.